Description
This French Onion Mushroom Casserole is a rich and comforting dish that combines caramelized pearl onions, sautéed crimini mushrooms, and a savory thyme-infused broth thickened with a buttery roux. Layers of garlic-buttered toasted baguette slices and cubes provide a delightful texture contrast, all topped with melted Swiss cheese. Perfect as a hearty side or vegetarian main, this casserole is baked to golden perfection and offers deep, complex flavors in every bite.
Ingredients
Scale
Vegetables
- 1¾ pounds pearl onions (peeled)
- 8 ounces crimini mushrooms, washed and thinly sliced
- 1½ tablespoons fresh thyme, finely chopped
- 1½ tablespoons garlic, minced, divided
Liquids
- 2 cups beef broth
- ½ cup dry red wine
- Olive oil for the pan
Dairy & Cheese
- 5 tablespoons unsalted butter, softened, divided
- 1½ cups Swiss cheese, grated, divided
Baking & Starches
- 2 tablespoons all-purpose flour
- 1 (approximately 2-foot) skinny French baguette
Seasonings
- Sea salt
- Freshly ground black pepper
Instructions
- Preheat the Oven: Set your oven to 375°F and adjust a rack to the top third to prepare for baking the casserole and toasting the bread.
- Peel and Caramelize the Onions: Peel the pearl onions and heat olive oil in a large sauté pan over medium heat. Add the onions and sauté, stirring occasionally, until they become deeply caramelized and sweet, about 15 to 20 minutes.
- Sauté Mushrooms and Add Thyme: Add more olive oil if needed and toss in the sliced mushrooms. Continue to sauté for about 10 minutes until they are caramelized. Stir in the chopped thyme and cook for 30 seconds until fragrant.
- Deglaze the Pan: Lower the heat, then pour in the beef broth and red wine to deglaze. Use a wooden spatula or spoon to scrape up the browned bits from the pan’s bottom, mixing them into the liquid. Bring the mixture to a boil over high heat, then reduce heat to low and simmer gently.
- Make a Roux and Thicken Sauce: In a small sauté pan, melt 2 tablespoons of butter over low heat. Whisk in the flour to create a roux. Gradually whisk this roux into the onion mixture. Bring it back to a boil briefly, then reduce to lowest heat and simmer uncovered for 15 minutes to thicken. Season to taste with salt and pepper.
- Prepare Bread Slices for Casserole Base: Slice about ¾ of the baguette diagonally into ½-inch thick pieces. Spread a mixture of 2 tablespoons softened butter and 1 tablespoon minced garlic onto each slice. Sprinkle with salt and pepper. Arrange on a baking sheet and bake in the preheated oven for 10 to 15 minutes until edges are golden. Transfer the toasted slices to the bottom of a 9x13x2-inch baking dish.
- Prepare Bread Cubes for Casserole Topping: Cut the remaining ¼ of the baguette into small cubes and place on the same baking sheet. Melt 1 tablespoon butter with the remaining ½ tablespoon garlic and drizzle over the cubes. Toss gently to coat evenly and season with salt and pepper. Bake for 10 to 15 minutes until golden. Set aside.
- Assemble the Casserole: Sprinkle ½ cup of grated Swiss cheese evenly over the toasted baguette slices in the baking dish. Pour the thickened onion and mushroom sauce over the bread. Top with the garlic-buttered toasted bread cubes and sprinkle the remaining 1 cup of Swiss cheese on top.
- Bake the Casserole: Place the assembled casserole in the preheated 375°F oven. Bake for approximately 20 minutes, or until the casserole is bubbling around the edges and the cheese is melted and golden.
- Cool and Serve: Allow the casserole to cool for about 10 minutes before serving. This resting time helps the flavors to meld and the casserole to set for easier serving.
Notes
- For peeling pearl onions quickly, blanching them briefly in boiling water before peeling can help loosen the skins.
- Using beef broth adds depth, but vegetable broth can be used for a lighter or vegetarian version.
- The garlic butter on the bread adds a flavorful crust and prevents sogginess from the sauce.
- Adjust seasoning, especially salt, after simmering since intensities can change during cooking.
- If Swiss cheese is unavailable, Gruyère or Fontina are good substitutes for melting and flavor.
- Make sure to spread bread slices and cubes evenly for consistent toasting.
