French Onion Mushroom Casserole

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 site, and in my kitchen.

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 was in culinary school that the process became more important to me - 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! (That said, a few shortcuts are below. ;-))

Don't rush. Cooking is not about time, it's 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.

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.

Ingredient Notes

Above, from top left: red wine, beef stock, Cremini mushrooms, pearl onions, thyme,garlic, French baguette, Swiss cheese, butter, flour, salt, pepper and olive oil.

• pearl onions - Any color works. I like to use a combination of white, red and yellow. You can also use frozen pearl onions or sliced yellow onions (see below).
• cremini mushrooms - Also called baby bella mushrooms. I love their earthy flavor in this dish. Substitution: button mushrooms.
• fresh thyme - You can substitute dried thyme. Use about ⅓ to ½ the amount.
• beef broth and dry red wine - These deglaze the pan and become the base of the rich sauce. (See Variations for a vegetarian option.)
• unsalted butter and all-purpose flour - These make the roux that thickens the sauce. The rest of the butter is mixed with garlic for the toasted bread.
• French baguette - A skinny one, about 2 feet long. Most of it is sliced for the bottom layer, and the rest is cubed for the croutons on top.
• garlic - Minced and mixed with butter for the bread.
• Swiss cheese (Emmental or Gruyère) - Grated, for the middle layer and the melted topping.
• olive oil, salt and pepper

Before we begin, a few notes about deglazing...

• 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 bottom of the pan, and it works its way back into the mix, adding an enormous amount of flavor with it.
• Don't deglaze the pan until you've properly browned the ingredients - this is when they're golden brown, and bits of food are (likely) stuck to the bottom of the pan.
• Sometimes, particularly if you're using a nonstick skillet, not much if anything will stick, and other times the entire bottom of the skillet might be browned and coated with pieces of food that need to be loosened. (We deglaze the pan either way!)

How to Make French Onion Mushroom Casserole

- Peel the onions. Click here for a step-by-step tutorial.

- Coat the bottom of a large skillet 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.

*If you're using frozen-thawed, peeled pearl onions, set the oven now.

- Set the oven. Preheat the oven to 375°F, and adjust a rack to the top third.

- Add a bit more oil to the pan, and add about half of the mushrooms. Stirring periodically, cook until they've shrunk enough so there's room for the second half. Add them, and cook until all of the mushrooms are nicely caramelized, about 10 minutes. Mix in the thyme and cook until it's very aromatic, about 30 seconds.

- Turn the heat to low and pour in the beef broth and wine to deglaze the pan. Use a flat-edged spatula 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 and simmer gently for about 15 minutes.

- While the mushroom-onion mixture is simmering, add 2 tablespoons of the butter to a small saucepan and melt it. With the heat on low, whisk in the flour. (This is a roux and will thicken the sauce.) Let it simmer very gently, stirring with a whisk every few minutes, until it's changed from a creamy-white color to a golden-brown.

- Once it's golden-brown, gently stir it into the mushroom-onion mixture, adding a little at a time. Keep stirring until it's blended in. 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 5 minutes. Season to taste with salt and pepper. Set aside.

- While the sauce is simmering, cut about ⅔ 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 2 teaspoons 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, about 10 minutes. Remove from the oven and add the toasted slices to the bottom of an (approximately) 9 x 13 x 2-inch baking dish. Set aside, and leave the oven on.

- Cut the remaining ⅓ of the baguette into small cubes (like croutons), and add them to the same baking sheet you used for the slices. Mix the remaining 2 tablespoons of butter with the remaining teaspoon 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, about 10 minutes. Remove from the oven and set aside. Leave the oven on.

- Sprinkle ½ cup of the cheese evenly over the toasted slices of bread in the baking dish. Pour the mushroom-onion mixture 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 in the (still) preheated 375°F oven until it's bubbling along the edges and the cheese is melted, about 15 minutes.

- Let it cool for about 10 minutes and then serve.

Recipe Tip: 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.

The order in which foods are layered can make a difference in how each bite feels in your mouth.

Variations

• Vegetarian version. If you'd like to make this dish vegetarian, use a hearty vegetable broth instead of the beef broth.
• Gluten-free version. Use gluten-free bread for both the bottom layer and the croutons, and a 1-to-1 gluten-free flour for the roux. If you can't find a gluten-free baguette, use your favorite gluten-free bread that's not too soft. Gelson's Markets carries gluten-free baguettes in the freezer section, and Whole Foods and other upscale or health food markets likely do too.
• Smaller batch. If you're cooking for 2 or 3, you can halve the recipe. It works well in a 9 x 9-inch baking dish.

Time-Saving Shortcuts

This is a time-consuming recipe. If you're like me, you'll enjoy the whole process and doing a lot of it from scratch, and I do think any recipe is at least a bit better with fresh ingredients. However, I realize I'm likely not in the majority, so below are a handful of shortcuts that save a lot of time - and it will still be delicious.

• Frozen pearl onions - They're already peeled. Just thaw them completely and pat them dry first.
• Pre-grated cheese
• Pre-sliced mushrooms - You still have to wash and dry them.
• Jarred or frozen minced garlic

Can You Use Regular Onions Instead of Pearl Onions?

Yes! Yellow onions work best. Slice them, but not too thinly, and cook them for the same amount of time as the pearl onions.

Serving Suggestions

This casserole is hearty enough to be a cozy main course with a crisp green salad. It's also a wonderful side dish for Thanksgiving or any holiday table, alongside roast turkey, beef or chicken.

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.
• You can assemble and bake it up to 4 hours before serving. Leave it at room temperature (don't refrigerate it). If it's been out of the oven for more than about 45 minutes, warm it in a 375°F oven just to heat it through, about 10 to 15 minutes, before serving.
• Another option is making the whole thing a day ahead, with the caveat that the bread will be softer. I actually slightly prefer it this way, but it's your call.

Can You Freeze It?

Yes! Let it cool to room temperature, then wrap it tightly, either whole or, even better, in individual portions. Freeze for up to about 1 month. The day before you want to serve it, move it to the refrigerator to thaw. Then let it sit on the counter for about 2 hours before reheating it in a 375°F oven until heated through, about 15 minutes for the whole casserole (less for individual portions).

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 Must-Try Comforting Casseroles

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.

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