French Onion Mushroom Casserole Recipe - It's like the best bowl of French Onion Soup you've ever had, only in the form of a casserole. It's the top of the soup -- the perfectly toasted French bread, the melting Swiss cheese, the caramelized onions, and all of the hearty, warm flavors of beef, garlic and mushrooms.

French Onion Mushroom Casserole Recipe

Imagine the best bowl of French Onion Soup you've ever had, only in casserole form.

*Peeling pearl onions can be very easy if you follow this step-by-step photographic guide.

Course Main Course
Cuisine American, French
Keyword make ahead casseroles
Prep Time 20 minutes
Cook Time 1 hour
Total Time 1 hour 20 minutes
Servings 6
Calories 458 kcal
Author Valentina K. Wein


  • olive oil for the pan
  • 1 3/4 pounds pearl onions (click here for How to Peel Pearl Onions)
  • 8- ounces Crimini mushrooms, washed and dried, thinly sliced
  • 1 1/2 tablespoons thyme, finely chopped
  • 2 cups beef broth
  • 1/2 cup dry red wine
  • 5 tablespoons unsalted butter, softened, divided
  • 2 tablespoons all-purpose flour
  • 1 2 foot, skinny French baguette
  • 1 1/2 tablespoons garlic, minced, divided
  • 1 1/2 cups Swiss cheese grated, divided
  • sea salt and freshly ground black pepper


  1. Preheat the oven to 375 degrees F, and adjust a rack to the top third.
  2. Once you've peeled the onions (here's how) -- coat the bottom of a large sauté pan with olive oil and place it over medium heat.
  3. Add the peeled onions and sauté them, stirring periodically, until they're nicely caramelized, 15 to 20 minutes.
  4. 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.
  5. Turn he 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.
  6. 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.)
  7. Using a whisk, mix the roux into the onions. 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.)
  8. While the sauce is simmering, cut about about 3/4 of the baguette into thin slices, diagonally -- about 1/2-inch thick. Place the slices on a baking sheet and set aside.
  9. 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 degree 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.
  10. Cut the remaining 1/4 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 1/2 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 degrees F), until golden, 10 to 15 minutes. Remove from the oven and set aside. Leave the oven on.
  11. Sprinkle 1/2 cup of the cheese evenly over the toasted slices of bread in the baking dish.
  12. Pour the onion sauce evenly on top and then evenly distribute the cubed, toasted bread on top of that.
  13. Sprinkle evenly with the remaining 1 cup of cheese.
  14. Bake in the (still) preheated 375 degree F oven until it's bubbling along the edges, about 20 minutes.
  15. Let it cool for about 10 minutes and then serve.