If you’re looking for a seriously cozy crowd-pleaser that’s as irresistible for weeknights as it is for holiday tables, this French Onion Mushroom Casserole Recipe is absolutely calling your name. Imagine the deep savory sweetness of caramelized onions, juicy mushrooms, and creamy cheese all baked to bubbling, golden perfection. You get layers of flavor in every bite, along with a crispy cracker topping that adds just the right amount of crunch. Whether you’re feeding your vegetarian friends or looking for a rich, hearty side dish, this casserole is destined to become a favorite for years to come.
Ingredients You’ll Need
What I truly love about the ingredients for this casserole is that every single one packs a punch of flavor or contributes to that luscious, craveable texture. Each component is simple, but together, they create magic you’ll taste in every forkful. Here’s what you’ll need:
- Unsalted butter: For deeply rich flavor and the perfect base for caramelizing onions.
- Olive oil: Keeps the butter from burning and adds a subtle, fruity note.
- Yellow onions: The star of the show—caramelized, they become sweet and jammy, forming the casserole’s flavor backbone.
- Sugar: Just a pinch is all it takes to help those onions caramelize beautifully.
- Salt: Essential for seasoning every layer and drawing out the onions’ moisture.
- Cremini or button mushrooms: Their earthy, meaty texture soaks up all those flavors like a dream.
- Garlic: For that unmistakable mellow depth that makes everything taste better.
- Fresh thyme (or dried): Adds subtle herbal notes that make this dish sing.
- Black pepper: Balances the richness and amplifies the savoriness.
- Vegetable or beef broth: Deglazes the pan and infuses the filling with more flavor.
- Sour cream: Lends luxurious creaminess and a tiny bit of tang.
- Gruyère or Swiss cheese (shredded): For a classic French onion vibe and gooey, melted richness.
- Mozzarella cheese (shredded): Bubbles up beautifully and binds everything together.
- Parmesan cheese (grated): Completes the cheese trio with nutty, salty notes.
- Crushed butter crackers or panko breadcrumbs: Adds that crave-worthy, crispy golden finish.
- Chopped fresh parsley (optional): For a pop of fresh green and an herby touch right before serving.
How to Make French Onion Mushroom Casserole Recipe
Step 1: Caramelize the Onions
This is where the magic begins! Start by melting your butter and olive oil together in a large skillet over medium heat. Tumble in those thinly sliced onions, sprinkle with salt and sugar, and let them cook low and slow. Stir them often for about 20 to 25 minutes—you want them deeply golden and almost jammy for that classic French onion flavor.
Step 2: Sauté the Mushrooms
Once the onions are caramelized, it’s time to add the sliced mushrooms. Give them 8 to 10 minutes over the same heat, stirring now and then, until they’re soft, browned, and have soaked up all that oniony goodness. This step adds heartiness to the French Onion Mushroom Casserole Recipe.
Step 3: Add Garlic and Thyme
Stir in the minced garlic, fresh (or dried) thyme, and black pepper. Cook for just about 1 minute—enough to release the amazing aromas without letting the garlic brown.
Step 4: Deglaze and Add Creaminess
Pour in the broth to deglaze the pan, scraping up all the flavorful browned bits stuck to the skillet (that’s pure flavor gold right there). Reduce the heat, then stir in the sour cream and half of the shredded Gruyère and mozzarella. Mix until everything is smooth, melty, and wonderfully creamy.
Step 5: Assemble the Casserole
Transfer the onion-mushroom-cheese mixture into a greased 8×8-inch (or similar size) baking dish. Top with the remaining shredded cheeses, sprinkle the Parmesan evenly, and finish with a generous layer of crushed crackers or panko for that signature crunch.
Step 6: Bake Until Golden
Pop the dish into your preheated oven at 375°F (190°C) and bake for about 20 to 25 minutes. You’re looking for a bubbling, golden-brown topping that makes your whole kitchen smell incredible. Let it cool for a few minutes before you dig in, then shower with fresh parsley if you like.
How to Serve French Onion Mushroom Casserole Recipe
Garnishes
You can never go wrong with a sprinkle of fresh parsley—it adds brightness and a hit of color that really sets off the rich, cheesy layers beneath. For a little extra flair, try a dusting of extra Parmesan or even a scatter of fresh thyme leaves.
Side Dishes
This casserole plays beautifully alongside everything from roasted meats to a simple green salad. It’s also at home next to grilled chicken or as the star attraction in a vegetarian spread. A slice of crusty bread on the side makes it even more delightful, ready to help you scoop up every last bit.
Creative Ways to Present
If you want to wow your dinner guests, consider baking the French Onion Mushroom Casserole Recipe in individual ramekins for a personal, elegant touch. Or, serve family-style at the table in a rustic ceramic dish and let everyone dig in together. Even leftover scoops make fabulous sandwich fillings or toast toppers for lunch!
Make Ahead and Storage
Storing Leftovers
Allow any leftovers to cool completely, then transfer them to an airtight container. Store in the fridge for up to 3 days, and be prepared for the flavors to deepen and meld after some time in the fridge—sometimes leftovers taste even better!
Freezing
This casserole freezes remarkably well. After baking, let it cool fully, then cover tightly with foil or plastic wrap and freeze for up to 2 months. Thaw overnight in the refrigerator before reheating for best results.
Reheating
To reheat, bake leftovers in a 350°F (175°C) oven until hot and bubbly, about 15–20 minutes. If you’re short on time, microwave individual portions until heated through, but baking will restore some of that irresistible crispy topping.
FAQs
Can I use a different type Side Dish
Absolutely! While cremini or button mushrooms are classic, feel free to use what you love—portobello, shiitake, or even a mix for more depth.
Is there a dairy-free way to make this French Onion Mushroom Casserole Recipe?
Yes! Substitute with dairy-free butter, plant-based sour cream, and your favorite vegan cheeses—the textures and cozy flavors will still shine through.
Can I prepare the casserole in advance?
You bet. Assemble the full casserole (without baking) up to a day in advance, keep covered in the fridge, and bake when you’re ready—just add a few minutes to the baking time if starting from cold.
What protein would pair well with this dish?
This casserole pairs beautifully with roast chicken, steak, or even salmon, but it’s hearty enough to stand on its own if you’d rather keep things vegetarian.
Can I swap the cracker topping for something else?
If you don’t have butter crackers, swap in panko breadcrumbs, crushed pretzels, or even fried onions for extra crunch and flavor on your French Onion Mushroom Casserole Recipe.
Final Thoughts
I can’t wait for you to try this French Onion Mushroom Casserole Recipe for yourself! Whether you love it as a weeknight comfort dish or the star of your next festive dinner, it’s guaranteed to turn any meal into something a little more special. Let this family favorite warm your table—and your heart—again and again.
Print
French Onion Mushroom Casserole Recipe
- Total Time: 45 minutes
- Yield: 6 servings 1x
- Diet: Non-Vegetarian
Description
This French Onion Mushroom Casserole is a delicious and comforting dish that combines caramelized onions, savory mushrooms, and cheesy goodness. Perfect as a vegetarian main or a hearty side dish.
Ingredients
For the caramelized onions:
- 2 tablespoons unsalted butter
- 2 tablespoons olive oil
- 2 large yellow onions, thinly sliced
- 1/2 teaspoon sugar
- 1/4 teaspoon salt
For the mushroom mixture:
- 16 oz cremini or button mushrooms, sliced
- 2 cloves garlic, minced
- 1 teaspoon fresh thyme or 1/2 teaspoon dried thyme
- 1/4 teaspoon black pepper
- 1/2 cup vegetable broth or beef broth
- 1/2 cup sour cream
- 1/2 cup shredded Gruyère or Swiss cheese
- 1/2 cup shredded mozzarella cheese
- 1/4 cup grated Parmesan cheese
- 1 cup crushed butter crackers or panko breadcrumbs for topping
- Chopped fresh parsley for garnish (optional)
Instructions
- Preheat the oven: Preheat the oven to 375°F (190°C).
- Caramelize the onions: In a large skillet, melt butter with olive oil over medium heat. Add sliced onions, salt, and sugar. Cook for 20–25 minutes until deeply caramelized.
- Cook the mushroom mixture: Add mushrooms to the skillet and cook for 8–10 minutes. Stir in garlic, thyme, and black pepper.
- Deglaze the pan: Add broth to the skillet, then stir in sour cream and half of the shredded cheeses until melted.
- Assemble the casserole: Transfer the mixture to a baking dish. Top with remaining cheeses and breadcrumbs.
- Bake: Bake for 20–25 minutes until bubbly and golden on top. Garnish with parsley before serving.
Notes
- This casserole is a great vegetarian main or side dish.
- For a richer version, use heavy cream instead of sour cream.
- Pair with roasted meats or a green salad for a complete meal.
- Prep Time: 20 minutes
- Cook Time: 25 minutes
- Category: Side Dish
- Method: Baking
- Cuisine: French-Inspired
Nutrition
- Serving Size: 1 portion
- Calories: 280
- Sugar: 5g
- Sodium: 420mg
- Fat: 20g
- Saturated Fat: 10g
- Unsaturated Fat: 9g
- Trans Fat: 0g
- Carbohydrates: 16g
- Fiber: 2g
- Protein: 9g
- Cholesterol: 40mg
