French Onion Mushroom Casserole is essentially the best bowl of French Onion Soup you've ever had, only in casserole form. The perfectly toasted French bread, the melting Swiss cheese, the caramelized onions, and all of the hearty, warm flavors of beef, garlic and mushrooms -- they're all what make this one of the best comfort foods ever!
Creative and comforting food for casual entertaining.
That's what I do -- here on my blog, and in my kitchen. I love cooking for others, but the "entertaining" part could be switched out for "eating," if your prefer. 😉
This is one of the best mushroom casserole recipes I've ever had, and so much is about the cooking process.
While I always loved cooking, it wasn't until I studied it in Culinary school, that the process became more important to me.
Creating the best comfort foods is more than just putting ingredients together -- it's all about how they're put together.
This French Onion Mushroom Casserole recipe is the perfect example of this.
Learning how and when to add certain ingredients to a pan, what to finish with and why -- it all fascinated me. And I learned patience is imperative.
You'll need patience for this French Onion Mushroom Casserole recipe, and I promise you, it'll be worth every second!
The Process
- Do not deglaze the pan until you've properly browned the ingredients.
Ingredients are properly browned when they're golden, and bits of food are stuck of the bottom of the pan. Deglazing is when you add a liquid to a very hot pan where you've browned food -- the liquid and the heat loosen anything stuck to the pan, and it works its way back into the mix, adding an enormous amount of flavor with it.
(In this recipe, it's the pearl onions and mushrooms we're browning, and the deglazing liquids are beef broth and red wine. )
- Wait several seconds after adding herbs, before moving on with the dish.
In this casserole, we want the flavor of the thyme to release before "burying" it with the next ingredients we add.
- Give the sauce time to thicken properly.
Wait for a sauce to bubble after adding a thickener -- this is how you will know what consistency you've reached, whether or not you might need more thickener. 
- Layer casserole ingredients carefully.
The order in which foods are layered can make a difference in how each bite feels in your mouth.
- Don't pull things from the oven too quickly -- it's not about time, it about the aroma, appearance and texture.
Sure, a quick and easy dinner is essential on some nights, but when time allows, I could spend all day in the process.
The entire process begins with an idea, of course. In this case, the idea for this onion mushroom casserole was born when Melissa's Produce delivered a huge bundle of gorgeous multi-colored pearl onions to my door.
Both the pearl onions and the mushrooms are the stars of this dish.
Recipe Tips and Substitutions
- Here's a guide for How to Easily Peel Pearl Onions. This will help you save a lot of time.
- While I love doing things from scratch, you can purchase already peeled pearl onions. These are typically found in the freezer section of the market. Be sure they're completely thawed before using them.
- Vegetarian? If you'd like to make this dish vegetarian, use a very hearty vegetable broth instead of the beef broth.
- If the bread on top gets nice and toasted before the cooking time is up, lightly cover the pan with foil until it's done.
Can you make French Onion Mushroom Casserole ahead?
- You can prepare all of the ingredients the day before you want to serve this casserole, including cooking the onion mixture.
- Assemble and bake up to 4 hours and at least 45 minutes before you're ready to serve. If you bake it earlier than 45 minutes ahead, do not refrigerate it before warming it in the oven just before serving. (Warm it in a 375°F oven just to heat it through, about 10 minutes.)
I absolutely love this French Onion Mushroom Casserole Recipe and I think you'll agree that it's one of the best comfort foods of all time.
More comforting casserole recipes:
- Hatch Chile Cornbread Casserole
- Smoky Spanish Smashed Potato Casserole
- Gruyère-Spinach Potatoes Au Gratin
This is not a sponsored post. Melissa's sent me the pearl onions for recipe testing. As always, all opinions are my own and I only write about things I truly love.
French Onion Mushroom Casserole Recipe
Ingredients
- olive oil for the pan
- 1¾ pounds pearl onions (click here for How to Peel Pearl Onions)
- 8 ounces Crimini mushrooms, washed and dried, thinly sliced
- 1½ tablespoons thyme, finely chopped
- 2 cups beef broth
- ½ cup dry red wine
- 5 tablespoons unsalted butter, softened, divided
- 2 tablespoons all-purpose flour
- 1 (approximately 2 foot) skinny French baguette
- 1½ tablespoons garlic, minced, divided
- 1½ cups Swiss cheese, grated, divided
- sea salt and freshly ground black pepper
Instructions
- Set the oven. Preheat the oven to 375°F, and adjust a rack to the top third.
- Peel and cook the onions. Peel the onions (here's how). Then coat the bottom of a large sauté pan with olive oil and place it over medium heat. Add the peeled onions and sauté them, stirring periodically, until they're nicely caramelized, 15 to 20 minutes.
- Add the mushrooms and thyme to the onions and cook. Add a bit more oil to the pan, and add the mushrooms. Again, stirring periodically, sauté until they're nicely caramelized, about 10 minutes. Mix in the thyme and cook until it's very aromatic, about 30 seconds.
- Deglaze the pan. Turn the heat to low and pour in the beef broth and wine to deglaze the pan. Use a wooden spatula (if you have one -- otherwise a spoon), to gently scrape any bits of stuck onion and/or mushroom from the bottom of the pan, and incorporate it into the liquid. Turn the heat to high and bring to a boil. Then reduce the heat to low again and simmer.
- Make a roux and add it to the onion mixture. Meanwhile, add 2 tablespoons of the butter to a small sauté pan and melt it. With the heat on low, whisk in the flour. (This is a roux and will thicken the sauce.) Using a whisk, mix the roux into the onion mixture. Again, turn the heat to high and bring to a boil. Then immediately reduce the heat to the lowest setting and let this simmer, uncovered, for about 15 minutes. Season to taste with salt and pepper. (Here's How to Season to Taste.)
- Prepare slices of the bread for the bottom of the casserole. While the sauce is simmering, cut about about ¾ of the baguette into thin slices, diagonally -- about ½ inch thick. Place the slices on a baking sheet and set aside. In a small bowl, mix 2 tablespoons of the butter with 1 tablespoon of the garlic and spread each slice of bread with it. Sprinkle with salt and pepper and place the baking sheet in the preheated 375°F oven until the edges are golden, 10 to 15 minutes. Remove from the oven and add the now toasted slices to the bottom of an (approximately) 9 x 13 x 2-inch baking dish. Set aside, and leave the oven on.
- Prepare cubes of the bread for the top of the casserole. Cut the remaining ¼ of the baguette into small cubes and add them to the same baking sheet you used for the slices. Mix the remaining 1 tablespoon of butter with the remaining ½ tablespoon of garlic and melt it in the microwave, or a tiny sauté pan. Drizzle this over the cubed bread on the baking sheet and sprinkle with salt and pepper. Toss gently with your hands to be sure it's well mixed. (Be careful as the baking sheet might still be hot!) Place this in the oven (still at 375°F), until golden, 10 to 15 minutes. Remove from the oven and set aside. Leave the oven on.
- Assemble. Sprinkle ½ cup of the cheese evenly over the toasted slices of bread in the baking dish. Pour the onion sauce evenly on top and then evenly distribute the cubed, toasted bread on top of that. Sprinkle evenly with the remaining 1 cup of cheese.
- Bake. Bake in the (still) preheated 375°F oven until it's bubbling along the edges, about 20 minutes.
- Cool and serve. Let it cool for about 10 minutes and then serve.
NOTES
NUTRITION
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Amanda
Wow. There is so much to love in this recipe! Caramelized onions, mushrooms, baguette slices, and swiss cheese in a casserole? If I hadn't already eaten dinner tonight, I'd be making this right now!
valentina
Thanks so much, Amanda! Hope you try it. Dessert? 😉
Tracy
Oh my gosh I am seriously drooling. I LOVE all of the flavors in this!!! Those onions look incredible!
valentina
Thanks, Tracy! I hope you give it a go. 🙂
Cathy | She Paused 4 Thought
Absolutely perfection!
valentina
Thank you my dear! 🙂 xo
Arthur in the Garden!
Yummy! All my favorites!
valentina
Thanks so much, Arthur. Hope you try it and love it!
Sandhya
What a great idea! Pinning the recipe. I love French onion soup!
valentina
Thanks for visiting -- and for the pin! Enjoy!
Helen
This looks and sounds just gorgeous. I would substitute vegetable stock for the beef broth to make it vegetarian. Delicious!
valentina
Perfect idea, Helen -- and you could totally mix it up and use potatoes instead of the bread for a gluten-free version. 🙂
linda
The more I cook the more I rely how the ingredients look and smell while cooking to know when a dish comes together. I suppose you could put something on a plate in 10 minutes or you can cook a really good meal.
valentina
Agreed, Linda. 🙂
Dana
This looks so good! I can tell just by looking at it that it's really savory and my mouth is pretty much watering...
valentina
Thank you, Dana! Hope you try it. It tastes as good as it looks. 🙂
Deb
What a fabulous casserole! This scrumptious recipe would make a great make ahead side dish for Thanksgiving or any other holiday event!
valentina
Thank you, Deb. Perfect idea to make it ahead for holiday dinners. 🙂 xo
Rachel
Mmmm... you got me at pearl onions and criminis. I'm thinking this would be a fabulous side this Thanksgiving!
valentina
Thanks Rachel! Yes, perfect along side the turkey. 🙂
Sarah
This sounds amazing! This sounds delicious! This sounds like I need to make it ASAP! XO
David
Wow - this sounds so fantastic, especially since I am in Chicago and freezing! What a brilliant recipe, Valentina!
valentina
You're the best, David. Bundle up and have some soup. 🙂 xo
Mandy
Made it and am now wondering if having it four times a week is excessive. So amazing, truly.
valentina
Oh, I'm so happy! Yay! Not excessive. Really. 😉 xo
Sharon
Ohhhhh, that's such a delicious and savory combination, but healthy too. Lovely!!
valentina
Thank you, Sharon. Enjoy!
Kathryn
Wow - this looks amazing - all of the yummiest parts of French onion soup in a casserole!! Definitely on my "must make" list!
valentina
Thanks, Kathryn. Hope you try it! 🙂
Boastful Food
Another home run! Loved the pic of the pearl onions simmering in the sauce. Yum!
valentina
Thanks so much, Raquel! XO
Cathy | She Paused 4 Thought
I have found pearl onions to be a labor of love that everyone appreciates. I am making this one for sure.
valentina
I'm so with you, Cathy. LOVE them. Enjoy! And Happy, happy Thanksgiving to you! XO
Julie
I made this for a pot luck I went to yesterday - it was quite the labor or love - was absolutely delicious!!! Although next time I would rather cut the labor and use frozen pearl onions - has anyone made this dish with them?
valentina
Hi Julie, you could absolutely use frozen peeled onions -- just make sure they are well thawed and drained of any excess moisture before you begin. So happy it was a hit! Thanks for sharing. 🙂
Jen
I used frozen pearl onions with this recipe and it turned out beautifully! I was in a rush and didn't even thaw them first; I caramelized them right from frozen and there wasn't any excess water in the pan. If there had been, I would have drained it off before proceeding with the mushrooms. Next time I will thaw them first though since ingredients can vary. I also saved time and an extra step by putting both the bread slices and cubes on a large cookie sheet at the same time and placing them under the broiler to toast. This recipe was loved by my dinner party guests last night and I will definitely be making it again!
valentina
Hi Jen! thanks so much for sharing! So happy it was a hit and that you were able to make it with your great time saving tips! Yay! Happy Holiday season to you! 🙂
Jenna
Hi! I want to make this for easter tomorrow! I am worried that it will get soggy tho if made ahead of time... does it keep well for a bit of time?
valentina
Yes, Jenna -- you can definitely make this ahead of time! You can get it all assembled tonight and put it in the oven tomorrow. OR, you can bake it tonight, letting it cool completely and then refrigerate. Take it out of the refrigerator in the morning, at least 30 minutes before serving, and place it in the oven again (375 degrees F) to heat it through, about 15 to 20 minutes. Serve! Enjoy!
Lori
Very delicious. I didn't wait until the onions were deeply caramelized but it was still good. I toasted the bread slices and bread cubes together at same time and I drizzled melted butter with garlic over it and then s/p and tossed. I didn't make a roux with flour and butter but I did add cornstarch. Didn't have fresh thyme so I used dried. So delicious! Thanks!
valentina
Hi Lori, I'm so happy you thought this dish was delicious. It's great you liked your tweaks to the recipe -- I often switch out dried thyme for fresh if I don't have it. (I have a tiny area in my garden for herbs, but I just planted new thyme and it's too small to pick.) I hope you keep making this recipe and loving it! Thanks for sharing how it went, for visiting my site and trying the recipes. Much appreciated. 🙂 ~Valentina
Elaine Geiger
I made this for dinner last night and it turned out great. My friend loved it! Next time I will use frozen Pearl onions to cut down on the prep time. Another note, my bread cubes on top were super toasted, almost burnt. So next time I will make smaller cubes so the crunch factor is just a little less. I ran out of red wine so used I used cooking Sherry and it worked well.
valentina
Hi Elaine, I'm so happy you loved this recipe. (And your friend!) I bet the sherry was delicious -- it's always a great match with mushrooms. 🙂 You can also cover the pan -- toward the later part of the cooking time -- lightly with foil, to avoid the bread getting too dark. And using the frozen onions is a good way to save time. So yay all around. I hope you have a great weekend and thanks. ~Valentina
Liz
Oh, boy, this looks like a delicious holiday side dish!! Adding mushrooms make it irresistible!!
valentina
Thanks so much, Liz. Hope you had a lovely holiday! 🙂 ~Valentina
David @ Spiced
Like you, I've always been fascinated by the process of cooking. And I also love cooking for others! Switching over to this recipe, I love it!! I've always loved French onion soup, but to mix it up and turn it into a casserole is such a great idea. I'm totally keeping this in mind for a cold winter day!
valentina
Thanks so much, David. Yes, perfect for a cold winter day! I wish we'd get a few more of those here in LA! 😉 ~Valentina
Kathy @ Beyond the Chicken Coop
I am a huge fan of French onion soup, so count me in for this casserole! All those mushrooms and onions...so much flavor. This is on my must-bake list!!!
valentina
Thanks, Kathy! Hope you love it! 🙂 ~Valentina
Karen (Back Road Journal)
This reminds me of a mushroom bread pudding recipe that I have only way better. 😁
valentina
Ooooh, mushroom bread pudding sounds fabulous! Thanks for more mushroom inspiration! 🙂 ~Valentina
Jackie
So good! I know the pearl onions are supposed to be the star of the show, but I couldn’t find them in my local grocery. I used regular onions. It turned out great! Thank you!
valentina
Hi Jackie! Thank you for writing in. I'm so happy you liked this recipe so much, and I'm sure it was just as delicious with the other onions. Thanks so much for visiting my site and checking out my recipes. 🙂 ~Valentina
Liz H
This was AMAZING! One of my best recipes. Will make again and again as a top recipe. Plant based as well.
valentina
Hi Liz, Thanks so much for writing in. I'm thrilled you loved this casserole. I always love it most this time of year -- it's such a hearty, cozy dish. Enjoy and thank you again. Warmly, Valentina
John
Absolutely Tasty Casserole, My Kitchen smelt heavenly during all the prep . I make it for a few lunches & Share the rest @ Work ,Thank you
Valentina
You made my day, John. Thank you! So happy you love it. 🙂 ~Valentina
Peggy
This looks divine! I’m thinking of making it for Thanksgiving, but my daughter is “no carb”. I’ll probably use the baguette on top as she can easily eat around them, but do you have a suggestion for the bottom? Do you think it would be ok without the bread on the bottom?
Valentina
Hi Peggy. Thanks for writing in. While I haven't tested it this way, I think it could work. I would however replace the bread with something, since it accounts for a considerable amount of the recipe. You could use more mushrooms (1 to 1.5 pounds instead of 1/2-pound), maybe a cup or two of cooked lentils, or if your daughter is okay with "low carb," wild rice (it's the lowest carb rice). It will a be a bit saucier without the bread on the bottom to soak up the juices. The flavors should still be delicious. I hope this helps! Happy early Thanksgiving! 🙂 ~Valentina