Description
This French Onion Mushroom Casserole is a hearty and comforting dish featuring caramelized pearl onions and sautéed Crimini mushrooms in a rich, savory sauce. Topped with toasted garlic baguette slices and cubes, layered with melted Swiss cheese, this casserole bakes to a bubbling golden finish, perfect for a cozy family meal or dinner party side.
Ingredients
Scale
Vegetables and Herbs
- 1¾ pounds pearl onions, peeled
- 8 ounces Crimini mushrooms, washed, dried, and thinly sliced
- 1½ tablespoons thyme, finely chopped
- 1½ tablespoons garlic, minced, divided
Liquids
- 2 cups beef broth
- ½ cup dry red wine
Dairy and Fats
- 5 tablespoons unsalted butter, softened, divided
- 1½ cups Swiss cheese, grated, divided
Baking and Seasoning
- 2 tablespoons all-purpose flour
- Olive oil, as needed for sautéing
- Sea salt, to taste
- Freshly ground black pepper, to taste
Bread
- 1 skinny French baguette (approximately 2 feet long)
Instructions
- Preheat Oven: Preheat the oven to 375°F and adjust a rack to the top third position.
- Peel and Caramelize Onions: Peel the pearl onions. Heat olive oil in a large sauté pan over medium heat. Add onions and sauté, stirring occasionally, until caramelized, about 15 to 20 minutes.
- Add Mushrooms and Thyme: Add more olive oil if needed, then add sliced mushrooms to the pan. Sauté until mushrooms are caramelized, about 10 minutes. Stir in thyme and cook for about 30 seconds until fragrant.
- Deglaze Pan: Reduce heat to low, pour in beef broth and red wine, scraping the bottom of the pan with a wooden spatula to loosen browned bits. Increase heat to high to bring to a boil, then reduce to low and simmer gently.
- Make Roux and Thicken Sauce: In a separate small sauté pan, melt 2 tablespoons of butter over low heat. Whisk in flour to create a roux. Mix the roux into the onion and mushroom mixture. Bring to a boil over high heat, then reduce heat to very low and simmer uncovered for 15 minutes. Season with salt and pepper to taste.
- Prepare Toasted Bread Slices: Cut about ¾ of the baguette into ½-inch thick diagonal slices. Mix 2 tablespoons of softened butter with 1 tablespoon minced garlic. Spread the garlic butter on bread slices, season with salt and pepper, and toast in the preheated oven for 10 to 15 minutes until edges are golden. Remove and arrange slices evenly in a 9×13-inch baking dish.
- Prepare Toasted Bread Cubes: Cut remaining ¼ of the baguette into small cubes. Mix remaining 1 tablespoon butter with remaining ½ tablespoon garlic and melt. Drizzle over the bread cubes on the baking sheet, season with salt and pepper, and toss to coat. Bake in the oven until golden, about 10 to 15 minutes. Remove and set aside.
- Assemble Casserole: Sprinkle ½ cup of grated Swiss cheese evenly over the toasted bread slices in the baking dish. Pour the simmered onion and mushroom sauce evenly over the cheese and bread slices. Distribute the toasted bread cubes over the top, then sprinkle evenly with remaining 1 cup of Swiss cheese.
- Bake: Bake the casserole in the preheated 375°F oven until bubbling around the edges and cheese is melted and slightly browned, about 20 minutes.
- Cool and Serve: Allow the casserole to cool for about 10 minutes before serving to set slightly.
Notes
- Pearl onions can be peeled easily by blanching them in boiling water for a minute, then shocking in ice water and gently squeezing out the skins.
- If you prefer, substitute beef broth with vegetable broth for a lighter taste.
- The red wine adds depth to the sauce; a dry variety works best for balancing flavors.
- Use a sharp Swiss cheese for optimal melt and flavor.
- Make sure to toast the bread slices and cubes separately to get the perfect texture for layering and topping.
- For an extra crispy topping, broil the casserole for the last 2 minutes, watching carefully to avoid burning.
