If you’re craving a dish that combines the deep, savory richness of caramelized onions with the earthy goodness of mushrooms, then this French Onion Mushroom Casserole Recipe is your new best friend. This soul-warming casserole delivers layers of intense flavor and a perfectly crispy cheesy topping that will have you coming back for seconds, whether it’s a cozy weeknight dinner or a special occasion. The harmony of sweet onions, tender mushrooms, and melted Gruyère cheese makes this dish truly unforgettable and surprisingly simple to prepare.
Ingredients You’ll Need
Gathering just the right ingredients is key to nailing the delicious layers in this French Onion Mushroom Casserole Recipe. Each ingredient plays a crucial role, from creating rich caramelized onions to adding creamy, cheesy depth and a crunchy topping that makes every bite delightful.
- Butter: Essential for caramelizing the onions to bring out their natural sweetness and add richness.
- Yellow onions: Thinly sliced, they form the flavor-packed base of the casserole with their deep caramelized notes.
- Olive oil: Helps cook the mushrooms perfectly and adds a subtle fruity undertone.
- Mushrooms: A mix of button or cremini mushrooms for earthy flavor and tender texture.
- Garlic: Minced fresh garlic adds a fragrant punch without overpowering the dish.
- Dried thyme: This herb provides an aromatic earthiness that complements both onions and mushrooms beautifully.
- Ground black pepper: Offers a mild heat that balances the overall creaminess.
- Vegetable broth: Creates a flavorful base for the sauce and keeps the casserole moist.
- Heavy cream: Adds luscious creaminess and helps bind all the ingredients together.
- Gruyère cheese: Melted and mixed in, it adds a savory, nutty depth.
- Parmesan cheese: Both mixed in and sprinkled on top for an extra sharp, salty kick.
- Fresh parsley: Optional, for a fresh green garnish that brightens the final presentation.
- Breadcrumbs: Preferably panko, for a delightfully crunchy, buttery topping.
- Melted butter: Coats the breadcrumbs to ensure a crispy, golden crust.
How to Make French Onion Mushroom Casserole Recipe
Step 1: Caramelize the Onions
Start by melting butter in a large skillet over medium heat. Add the thinly sliced yellow onions and cook them patiently for 15 to 20 minutes, stirring occasionally. You’ll want them to become golden brown and beautifully caramelized, releasing a deep sweetness that builds the foundation of this casserole. Keep an eye on them and add a splash of water if they start sticking to prevent burning while softening evenly.
Step 2: Cook the Mushrooms
Next, drizzle olive oil into the skillet with your caramelized onions. Toss in the sliced mushrooms and sauté for about 7 to 8 minutes. This step allows the mushrooms to release their moisture and develop a tender, juicy texture that pairs perfectly with the onions.
Step 3: Add Aromatics and Seasoning
Stir in the minced garlic, dried thyme, and freshly ground black pepper. Cook everything together for just another minute, letting the garlic become fragrant and the herbs infuse their subtle earthiness into the mixture.
Step 4: Create the Sauce
Pour in the vegetable broth to the skillet and allow the mixture to simmer gently for 2 to 3 minutes. This step melds all of the flavors, letting the onions, mushrooms, and herbs soak in awesome savory depth before thickening the sauce.
Step 5: Add Cream and Thicken
Stir in the heavy cream and continue simmering for another 3 to 4 minutes until the sauce thickens slightly into a creamy luscious base that will hold your casserole together beautifully.
Step 6: Assemble the Casserole
Preheat your oven to 375°F (190°C) to get it ready for baking. Transfer the savory onion and mushroom mixture into a greased 9-by-13-inch baking dish, spreading it evenly so every scoop is packed with flavor.
Step 7: Add the Cheeses
Sprinkle the shredded Gruyère and Parmesan cheeses over the hot mixture and give it a gentle stir. The cheeses melt into the creamy sauce adding nutty and sharp flavors that make this French Onion Mushroom Casserole Recipe irresistibly cheesy.
Step 8: Prepare and Add the Topping
Combine the breadcrumbs, melted butter, and additional grated Parmesan cheese in a small bowl. Mix well until the crumbs are evenly coated with butter, then spread this crunchy topping evenly over the casserole to create a golden crust during baking.
Step 9: Bake Until Golden and Bubbling
Pop the casserole into the preheated oven and bake for about 20 to 25 minutes. You’ll know it’s done when the top is beautifully golden brown and the casserole bubbles around the edges, releasing a mouth-watering aroma.
Step 10: Rest and Garnish
Once out of the oven, let your French Onion Mushroom Casserole Recipe rest for a few minutes to set. Finish with a sprinkle of freshly chopped parsley to add a lovely pop of color and freshness right before serving.
How to Serve French Onion Mushroom Casserole Recipe
Garnishes
Adding fresh parsley at the end is a simple touch that brightens the dish visually and adds fresh herbal notes. You can also sprinkle a little extra Parmesan cheese right before serving for an added savory hit that melts beautifully on top.
Side Dishes
This casserole shines as a main dish but also pairs brilliantly with crisp green salads, roasted vegetables, or crusty artisan bread to soak up every bit of the luscious sauce. For a heartier meal, serve alongside roasted chicken or a grilled steak.
Creative Ways to Present
For a special occasion, consider serving the casserole in individual ramekins for a personalized touch. You can also accompany it with a drizzle of balsamic glaze or a sprinkle of toasted nuts like walnuts for an unexpected crunch and flavor contrast.
Make Ahead and Storage
Storing Leftovers
If you’re lucky enough to have leftovers, store them in an airtight container in the refrigerator. The casserole will stay delicious and can be enjoyed within 3 to 4 days without losing its wonderful flavors.
Freezing
This French Onion Mushroom Casserole Recipe freezes quite well. To freeze, cover it securely with plastic wrap and then foil or place in a freezer-safe container. It will keep for up to 2 months—perfect for prepping meals in advance.
Reheating
To reheat, thaw the casserole overnight in the fridge if frozen. Warm it in a preheated oven at 350°F (175°C) until heated through and crisp again on top, about 15 to 20 minutes. Microwave reheating is possible but might soften the crispy topping.
FAQs
Can I use other types of mushrooms in this casserole?
Absolutely! While button and cremini mushrooms work wonderfully, you can experiment with shiitake, portobello, or oyster mushrooms for different flavors and textures. Just adjust cooking times slightly if you use larger mushrooms.
Is it possible to make this vegetarian?
Yes, this recipe is naturally vegetarian if you use vegetable broth instead of chicken broth. It’s a hearty and satisfying option for vegetarians craving a rich, cheesy casserole.
Can I make this casserole dairy-free?
To make a dairy-free version, substitute the butter with a plant-based spread, use coconut or cashew cream instead of heavy cream, and try dairy-free cheese alternatives. Keep in mind, the flavor profile might vary slightly but will still be delicious.
How can I make the topping extra crunchy?
Using panko breadcrumbs and tossing them with melted butter before baking is a great way to get a perfectly crunchy top. You can also add a sprinkle of crushed nuts or seeds for added texture and flavor.
Can this casserole be doubled for a larger crowd?
Definitely! Just double the ingredients and use a larger baking dish. Adjust the baking time by keeping an eye on the golden-brown topping and bubbling edges to ensure it’s thoroughly heated.
Final Thoughts
There is something undeniably comforting and indulgent about this French Onion Mushroom Casserole Recipe. Every time I make it, it feels like a culinary hug—warm, flavorful, and effortlessly satisfying. Whether you’re looking for a new twist on classic French onion flavors or simply craving a cheesy, mushroomy delight, this casserole is an easy crowd-pleaser that’s sure to become one of your favorites. Give it a try and watch it disappear from the table!
Print
French Onion Mushroom Casserole Recipe
- Total Time: 55 minutes
- Yield: 6 servings 1x
- Diet: Vegetarian
Description
This delicious French Onion Mushroom Casserole combines caramelized onions and tender mushrooms in a creamy, cheesy sauce topped with a crunchy breadcrumb crust. Perfect as a hearty side dish or a comforting vegetarian main, it offers rich flavors with Gruyère, Parmesan, and a touch of thyme for depth. Baked to golden perfection, this casserole is an easy and crowd-pleasing recipe ideal for family dinners or special occasions.
Ingredients
Main Ingredients
- 2 tablespoons butter
- 2 large yellow onions, thinly sliced
- 1 tablespoon olive oil
- 4 cups mushrooms, sliced (button, cremini, or a mix)
- 3 cloves garlic, minced
- 1 teaspoon dried thyme
- ½ teaspoon ground black pepper
- 1 cup vegetable broth (or chicken broth)
- ½ cup heavy cream
- 1 cup shredded Gruyère cheese (or Swiss cheese)
- ½ cup shredded Parmesan cheese
- 1 tablespoon fresh parsley, chopped (optional, for garnish)
Topping
- 1 cup breadcrumbs (preferably panko for extra crunch)
- 2 tablespoons melted butter
- ¼ cup grated Parmesan cheese
Instructions
- Caramelize the Onions: In a large skillet, melt the butter over medium heat. Add the sliced onions and cook, stirring occasionally, for 15-20 minutes, or until the onions are golden brown and caramelized. If the onions start to burn, add a splash of water and continue to cook until they soften and caramelize.
- Cook the Mushrooms: Add the olive oil to the skillet with the onions. Add the sliced mushrooms and cook, stirring occasionally, for 7-8 minutes until the mushrooms release their moisture and become tender.
- Add Seasonings: Add the garlic, thyme, and black pepper to the skillet, cooking for another minute until the garlic becomes fragrant.
- Make the Sauce: Pour in the vegetable broth and let it simmer for 2-3 minutes, allowing the flavors to meld together.
- Thicken the Sauce: Stir in the heavy cream and bring the mixture to a simmer. Let it cook for another 3-4 minutes until the sauce thickens slightly.
- Preheat Oven: Preheat your oven to 375°F (190°C) to prepare for baking the casserole.
- Assemble the Casserole: Transfer the onion and mushroom mixture to a greased 9×13-inch baking dish.
- Add Cheese: Sprinkle the shredded Gruyère and Parmesan cheese over the mixture, stirring gently to combine the cheeses evenly.
- Prepare the Topping: In a small bowl, mix the breadcrumbs with melted butter and grated Parmesan until the breadcrumbs are well coated.
- Add Topping: Evenly sprinkle the breadcrumb mixture over the top of the casserole.
- Bake the Casserole: Place the casserole in the preheated oven and bake for 20-25 minutes, or until the top is golden brown and the casserole is bubbling.
- Rest and Serve: Remove the casserole from the oven and let it sit for a few minutes before serving.
- Garnish and Enjoy: Optionally garnish with chopped fresh parsley and serve the casserole hot.
Notes
- For best texture, use a mix of button and cremini mushrooms, or your favorite mushrooms.
- Gruyère cheese can be substituted with Swiss cheese if preferred.
- If you want a richer flavor, use chicken broth instead of vegetable broth.
- To make the casserole gluten-free, substitute breadcrumbs with gluten-free breadcrumbs.
- The casserole can be prepared a day ahead and baked fresh before serving.
- Store leftovers covered in the refrigerator for up to 3 days.
- Reheat in the oven to retain the crunchy topping rather than microwave.
- Prep Time: 15 minutes
- Cook Time: 40 minutes
- Category: Casserole
- Method: Baking
- Cuisine: French
