French Onion Mushroom Casserole Recipe
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!
Servings: 6 servings
Calories: 512kcal
Ingredients
- olive oil for the pan
- 1¾ pounds pearl onions peeled, see notes
- 1 pound cremini mushrooms washed and dried, thinly sliced
- 1½ tablespoons fresh thyme finely chopped, or dried (see notes)
- 2 cups beef broth
- ½ cup dry red wine
- 6 tablespoons unsalted butter softened, divided
- 2 tablespoons all-purpose flour
- 1 skinny French baguette about 2 feet long
- 1 tablespoon garlic minced, divided
- 1½ cups Swiss cheese grated, divided
- salt and freshly ground black pepper
Instructions
- Peel and cook the onions. Peel the onions (here's how). Then 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 the mushrooms and thyme to the onions and cook. 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 in the pan 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.
- Deglaze the pan. 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.
- Make a roux and add it to the onion mixture. 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. (Here's How to Season to Taste.) Set aside.
- Prepare slices of the bread for the bottom of the casserole. 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.
- Prepare cubes of the bread for the top of the casserole. 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.
- Assemble. 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. Bake in the (still) preheated 375°F oven until it's bubbling along the edges and the cheese is melted, about 15 minutes.
- Cool and serve. Let it cool for about 10 minutes and then serve.
Notes
Pearl onions: Peeling them is easy with this step-by-step guide. You can also use frozen peeled pearl onions (thaw them completely first), or sliced yellow onions (not too thinly sliced), cooked for the same amount of time.
Dried thyme: Substitute about ⅓ to ½ the amount of fresh.
Shortcuts: Use frozen pearl onions (thawed), pre-grated cheese, pre-sliced mushrooms, and jarred or frozen minced garlic.
Vegetarian option: Use a very hearty vegetable broth instead of the beef broth.
Gluten-free option: Use gluten-free bread and a 1-to-1 gluten-free flour for the roux.
Smaller batch: Halve the recipe for 2 or 3 people and use a 9 x 9-inch baking dish.
If the bread browns too quickly, loosely cover the dish with foil until it's done.
Make ahead: Prepare all of the components, including the onion mixture, the day before. Assemble and bake up to 4 hours before serving, and keep it at room temperature. Rewarm in a 375°F oven for 10 to 15 minutes. You can also make the whole casserole a day ahead; the bread will be softer.
To freeze: Cool completely, wrap tightly (whole or in portions) and freeze for up to about 1 month. Thaw in the refrigerator the day before, let it sit on the counter for about 2 hours, then reheat in a 375°F oven until heated through, about 15 minutes (less for portions).
Nutritional information is automatically calculated, so it should only be used as an estimate.
Nutrition
Calories: 512kcal | Carbohydrates: 45g | Protein: 17g | Fat: 29g | Saturated Fat: 13g | Polyunsaturated Fat: 2g | Monounsaturated Fat: 12g | Trans Fat: 0.4g | Cholesterol: 50mg | Sodium: 681mg | Potassium: 683mg | Fiber: 4g | Sugar: 9g | Vitamin A: 602IU | Vitamin C: 13mg | Calcium: 358mg | Iron: 3mg