French Onion Mushroom Casserole Recipe

Imagine the comforting flavors of French onion soup combined with the earthiness of mushrooms, all baked into a cheesy, savory casserole. This French Onion Mushroom Casserole is the ultimate side dish or hearty main course that’s perfect for cozy dinners and gatherings. It’s rich, warm, and incredibly satisfying with layers of sautéed mushrooms, onions, and a flavorful broth, topped with a golden, melty cheese crust. If you’re a fan of French onion soup, you’re going to love this casserole—it’s basically the soup, but in the form of a baked, gooey masterpiece!

Why You’ll Love French Onion Mushroom Casserole

This casserole is so much more than just a side dish. Here’s why it’s going to become a new favorite in your kitchen:

Comforting and Hearty

The combination of caramelized onions and mushrooms makes this casserole feel so rich and comforting. The savory flavors meld perfectly together, creating a dish that feels like a warm hug in every bite.

Full of Flavor

The depth of flavor from the sautéed onions, mushrooms, and broth makes this casserole anything but bland. With every forkful, you’ll get the delicious umami from the mushrooms and the sweetness from the onions.

Cheesy Goodness

What’s better than a casserole topped with melted cheese? The gooey, bubbly cheese on top adds a perfect finishing touch and brings everything together beautifully.

Versatile

Serve this casserole as a side dish to any meal, or enjoy it as a vegetarian main course. It’s great with roasted meats, or served alongside a simple salad for a lighter option.

Easy to Make

Despite the layered flavors, this casserole is super easy to put together. It requires just a few simple ingredients and comes together in one baking dish for minimal cleanup!

Ingredients

Here’s what you’ll need to create this savory, cheesy casserole:

For the Casserole:

• Mushrooms
  Fresh mushrooms (white, cremini, or a mix) provide an earthy base to the casserole. Their umami flavor is what really gives this dish its depth.
• Yellow Onions
  Sliced yellow onions are the perfect choice for caramelizing. They add sweetness and richness to the casserole as they cook down.
• Butter
  Butter helps to sauté the mushrooms and onions, making them extra tender and flavorful.
• Garlic
  Minced garlic adds a savory kick that deepens the flavor of the mushrooms and onions.
• Vegetable Broth
  Vegetable broth is used to create a flavorful base and to deglaze the pan after sautéing the vegetables. You can substitute with chicken broth if preferred.
• Heavy Cream
  Heavy cream adds richness and a velvety texture to the casserole, helping to bind everything together.
• Thyme
  Fresh thyme leaves bring a beautiful herbal note to the dish, enhancing the savory flavors of the mushrooms and onions.
• Bay Leaves
  Bay leaves give the casserole a subtle, aromatic flavor that rounds out the dish.
• Salt and Pepper
  Season to taste for the perfect balance of flavor.

For the Topping:

• Gruyère Cheese
  Gruyère cheese melts beautifully and adds a nutty, savory flavor that pairs perfectly with the caramelized onions and mushrooms.
• Parmesan Cheese
  Parmesan cheese adds a salty, savory finish and enhances the overall richness of the casserole.
• Bread Crumbs
  A sprinkle of breadcrumbs adds a satisfying crunch to the top of the casserole, making every bite a little more exciting.

(Note: the full ingredients list, including measurements, is provided in the recipe card directly below.)

Instructions

Let’s get cooking! Follow these steps to make your French Onion Mushroom Casserole:

Sauté the Mushrooms and Onions

In a large skillet, melt butter over medium heat. Add the sliced onions and cook, stirring occasionally, until they begin to soften and caramelize, about 10-12 minutes. Add the garlic and cook for another 1-2 minutes until fragrant. Add the sliced mushrooms to the skillet and cook until they release their moisture and become tender, about 8-10 minutes.

Make the Casserole Base

Once the mushrooms and onions are cooked down, pour in the vegetable broth and stir to combine. Add the heavy cream, thyme, and bay leaves, then season with salt and pepper to taste. Let the mixture simmer for another 5-7 minutes, allowing the flavors to meld together. Remove from heat and discard the bay leaves.

Assemble the Casserole

Preheat your oven to 375°F (190°C). Grease a baking dish with butter or cooking spray. Pour the mushroom and onion mixture into the prepared baking dish and spread it out evenly.

Add the Topping

In a small bowl, mix the grated Gruyère and Parmesan cheeses with the breadcrumbs. Sprinkle the cheese-breadcrumb mixture evenly over the top of the casserole.

Bake

Place the casserole in the oven and bake for 20-25 minutes, or until the top is golden and bubbly. The cheese should be melted, and the breadcrumbs should have a nice crispy texture.

Serve and Enjoy

Remove the casserole from the oven and let it cool for a few minutes before serving. Garnish with fresh thyme or parsley if desired, and enjoy this rich, comforting dish!

Nutrition Facts

Servings: 6
Calories per serving: 320 calories

Nutritional Breakdown (per serving):

• Calories: 320
• Total Fat: 22g
  • Saturated Fat: 13g
  • Trans Fat: 0g
• Cholesterol: 65mg
• Sodium: 630mg
• Total Carbohydrates: 15g
  • Dietary Fiber: 3g
  • Sugars: 6g
• Protein: 11g
• Vitamin A: 8%
• Vitamin C: 4%
• Calcium: 30%
• Iron: 6%

Preparation Time

• Prep Time: 15 minutes
• Cook Time: 35 minutes
• Total Time: 50 minutes

How to Serve French Onion Mushroom Casserole

This casserole is perfect as a side dish to complement any meal, but it also stands beautifully on its own as a vegetarian main course. Here are some ways to serve it:

As a Side Dish

Pair this casserole with roasted meats like beef, chicken, or pork for a hearty and satisfying meal.

With a Salad

Serve it with a simple green salad or a fresh arugula salad for a light and well-balanced dinner.

On Its Own

Enjoy this casserole on its own as a comforting, vegetarian main course. It’s filling and flavorful enough to be the star of the meal!

With Crusty Bread

For an extra indulgent touch, serve this casserole with a slice of warm, crusty bread to soak up all the delicious sauce.

Additional Tips

Caramelize the Onions Slowly

For the best flavor, take your time caramelizing the onions. The slower you cook them, the sweeter and richer they’ll become.

Make Ahead

This casserole can be assembled ahead of time and stored in the fridge for up to 24 hours. Just bake it when you’re ready to serve.

Add Protein

Want to make this casserole more substantial? Try adding cooked chicken, sausage, or even crispy bacon to the mixture before baking.

Gluten-Free Option

To make this casserole gluten-free, use gluten-free breadcrumbs and ensure your broth is gluten-free.

Use Different Mushrooms

Mix up the types of mushrooms used in this casserole! Try adding portobello, shiitake, or even oyster mushrooms for more variety in flavor and texture.

FAQ Section

Q1: Can I use frozen mushrooms for this recipe?
A1: Fresh mushrooms are preferred for this recipe, but if you only have frozen mushrooms, make sure to thaw and drain them thoroughly before using them.

Q2: Can I make this casserole in advance?
A2: Yes, you can assemble the casserole a day ahead of time and refrigerate it. When ready to serve, bake as directed.

Q3: Can I substitute the heavy cream?
A3: If you want a lighter version, you can substitute heavy cream with half-and-half or even milk. The texture may be a little lighter, but it will still be delicious.

Q4: Can I use a different type of cheese?
A4: Yes! You can substitute Gruyère with Swiss cheese, mozzarella, or cheddar for a different flavor. Parmesan works well for a salty, tangy topping.

Q5: Can I make this casserole dairy-free?
A5: You can make a dairy-free version by using plant-based cream and cheese substitutes.

Q6: How do I store leftovers?
A6: Store leftovers in an airtight container in the fridge for up to 3 days. Reheat in the oven at 350°F (175°C) until warmed through.

Q7: Can I freeze this casserole?
A7: Yes, you can freeze the assembled casserole before baking. Cover tightly with plastic wrap and foil, and freeze for up to 3 months. Bake directly from the freezer, but add an additional 10-15 minutes to the baking time.

Q8: How do I make the casserole spicier?
A8: Add some red pepper flakes or a pinch of cayenne pepper to the mushroom and onion mixture for a bit of heat.

Q9: Can I add other vegetables to this casserole?
A9: Absolutely! You can add vegetables like spinach, zucchini, or bell peppers for extra color and flavor.

Q10: Can I cook this in a slow cooker?
A10: While this recipe is best baked, you can cook the mushroom and onion mixture in a slow cooker on low for about 4 hours, then add the cheese and breadcrumbs before serving.

Conclusion

This French Onion Mushroom Casserole is the perfect dish to satisfy your craving for something rich, comforting, and full of flavor. The combination of sweet caramelized onions, tender mushrooms, and gooey cheese creates a truly indulgent experience. Whether you serve it as a side or enjoy it on its own, this casserole is sure to be a hit at your next dinner. Enjoy every cheesy, savory bite!

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