French Onion Mushroom Casserole Recipe
Imagine the most comforting, bubbling casserole filled with deeply caramelized onions, savory mushrooms, and a creamy cheese sauce—all topped with a golden, buttery cracker crust. That’s exactly what you get with this French Onion Mushroom Casserole Recipe. It’s the sort of dish that brings everyone running to the table, perfect for holidays, potlucks, or any time you need a little extra coziness on your plate. With a blend of Gruyère, mozzarella, and a hint of thyme, this casserole captures the soul of French onion soup with an irresistible, hearty twist. If you love layers of flavor and crave a side dish that could easily steal the spotlight, this recipe is about to become a favorite.
Ingredients You’ll Need
Ingredients You’ll Need
The magic of this French Onion Mushroom Casserole Recipe is how a handful of humble, everyday ingredients come together to create something truly spectacular. Each ingredient plays a special role, from the richness of the dairy to the umami depth of the mushrooms and onions—don’t skip a thing!
- Unsalted butter (2 tablespoons): Adds a luscious base for caramelizing the onions and brings out their natural sweetness.
- Olive oil (1 tablespoon): Blends with the butter for a perfectly balanced sauté and prevents burning.
- Onions, thinly sliced (2 large): The star of the show—slow-cooked until golden, they impart classic French onion flavor.
- Salt (1/2 teaspoon): Enhances all the savory elements and balances the sweetness of the onions.
- Black pepper (1/4 teaspoon): Provides a gentle background heat.
- Sugar (1 teaspoon): Helps the onions caramelize and develop a deep, rich color.
- Garlic, minced (3 cloves): Adds aromatic depth and a subtle kick.
- Cremini or button mushrooms, sliced (1 pound): Bring earthiness and meaty texture that pairs perfectly with the onions.
- Dried thyme (1/2 teaspoon): Offers a classic herbal note reminiscent of French onion soup.
- Worcestershire sauce (1 tablespoon): Sneaks in a little umami and complexity.
- Sour cream (1/2 cup): Makes the casserole creamy and tangy.
- Mayonnaise (1/4 cup): Adds richness and binds the mixture beautifully.
- Gruyère or Swiss cheese, shredded (1/2 cup): Melts into the casserole for nutty, gooey goodness.
- Mozzarella cheese, shredded (1/2 cup): Gives stretch and creaminess to every bite.
- Buttery crackers, crushed (1 cup): The ultimate crispy topping—think Ritz or similar.
- Melted butter (1 tablespoon, for topping): Ensures the cracker topping bakes up perfectly golden.
- Chopped parsley (optional): For a pop of color and freshness at serving time.
How to Make French Onion Mushroom Casserole Recipe
Step 1: Preheat and Prep
Start by preheating your oven to 375°F (190°C) and greasing a medium-sized casserole dish. Taking this simple step ensures nothing sticks and your French Onion Mushroom Casserole Recipe comes out of the dish beautifully golden and intact.
Step 2: Caramelize the Onions
In a large skillet, melt the unsalted butter together with the olive oil over medium heat. Add your thinly sliced onions, sprinkle in the salt, pepper, and sugar, and let the mixture cook, stirring every so often, for 15 to 20 minutes. The goal is to coax the onions into a deep golden brown without rushing—they should be sweet, soft, and irresistibly fragrant.
Step 3: Sauté Garlic and Mushrooms
Once your onions are beautifully caramelized, stir in the minced garlic and sliced mushrooms. Cook for another 8 to 10 minutes, allowing the mushrooms to release their moisture and take on a lovely brown color. This is when your kitchen will start to smell utterly amazing! Add the dried thyme and Worcestershire sauce, then remove the skillet from the heat.
Step 4: Mix the Creamy Base
In a large mixing bowl, combine the sour cream, mayonnaise, shredded Gruyère (or Swiss), and mozzarella cheese. This creamy, cheesy mixture is the heart of the casserole, binding everything together while adding tang and richness.
Step 5: Combine and Assemble
Add the warm onion and mushroom mixture to the bowl with the creamy cheeses. Give everything a good stir so it’s evenly combined, then transfer this glorious mixture into your prepared casserole dish, smoothing out the top with a spatula.
Step 6: Make and Add the Topping
In a small bowl, mix the crushed buttery crackers with the melted butter until every crumb is coated. Sprinkle this mixture evenly over the casserole. This will bake up into a crisp, golden topping that’s simply irresistible.
Step 7: Bake to Perfection
Slide your casserole into the oven and bake for 20 to 25 minutes, or until the top is bubbling and beautifully browned. If you like, garnish with a handful of chopped parsley for a burst of color before serving your French Onion Mushroom Casserole Recipe warm.
How to Serve French Onion Mushroom Casserole Recipe
Garnishes
A sprinkle of freshly chopped parsley adds a bright, herbal note and a pop of color that makes the casserole look even more inviting. You can also try a light dusting of extra Gruyère or a grind of black pepper on top right before serving for an extra touch of elegance.
Side Dishes
This French Onion Mushroom Casserole Recipe pairs beautifully with roasted or steamed vegetables, a simple green salad, or even a hearty grain like wild rice or quinoa. It’s also a fantastic companion to roasted chicken or turkey, making it a versatile side for holidays and weeknight dinners alike.
Creative Ways to Present
Serve the casserole in individual ramekins for a dinner party-worthy presentation, or spoon it generously onto toasted baguette slices for a crowd-pleasing appetizer. You can even turn leftovers into a decadent filling for stuffed bell peppers or savory crepes—the possibilities are endless!
Make Ahead and Storage
Storing Leftovers
Leftover French Onion Mushroom Casserole Recipe keeps beautifully in the refrigerator. Simply cover the dish tightly with foil or plastic wrap, or transfer portions to an airtight container. It will stay fresh for up to 3 days, perfect for easy lunches or side dishes throughout the week.
Freezing
To freeze, let the casserole cool completely, then wrap tightly with plastic wrap and a layer of foil. For best results, freeze without the cracker topping; add it fresh before baking. The casserole can be frozen for up to 2 months—just thaw overnight in the fridge before reheating.
Reheating
To reheat, cover the casserole loosely with foil and warm it in a 350°F (175°C) oven until heated through, about 20 minutes. For individual portions, a quick zap in the microwave works too. If the topping softens, pop it under the broiler for a minute or two to restore that irresistible crunch.
FAQs
Can I use different types of mushrooms?
Absolutely! While cremini or button mushrooms are classic, feel free to experiment with baby bella, shiitake, or even a mix for more texture and earthy flavor in your French Onion Mushroom Casserole Recipe.
Is there a way to make this casserole gluten-free?
Yes! Substitute gluten-free crackers for the topping and double-check that your Worcestershire sauce is gluten-free, and you’ll have a delicious gluten-free version of this comforting dish.
Can I prepare the casserole ahead of time?
You can assemble the casserole (without the cracker topping) up to a day in advance and refrigerate it. When ready to bake, add the topping and pop it in the oven—easy and ideal for busy schedules or holiday prep.
What can I use instead of mayonnaise?
If you’re not a fan of mayo, swap in plain Greek yogurt or extra sour cream for a similar creamy texture with a little extra tang.
Can I make French Onion Mushroom Casserole Recipe vegan?
With a few swaps—plant-based butter, vegan cheese, and dairy-free sour cream and mayo—you can absolutely transform this casserole into a vegan favorite while keeping all the flavor and comfort you love.
Final Thoughts
If you’re looking for a dish that’s rich in flavor, crowd-pleasing, and endlessly comforting, you truly can’t go wrong with this French Onion Mushroom Casserole Recipe. I hope you give it a try and let it become a new favorite in your kitchen—it’s one of those recipes you’ll want to come back to again and again!
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French Onion Mushroom Casserole Recipe
- Total Time: 45 minutes
- Yield: 6 servings 1x
- Diet: Vegetarian
Description
This French Onion Mushroom Casserole is a creamy, cheesy, and comforting side dish perfect for holidays or weeknight dinners. Featuring caramelized onions, sautéed mushrooms, and a rich blend of Gruyère and mozzarella cheeses, it’s topped with buttery crushed crackers for a delightful crunch. This vegetarian casserole can also serve as a hearty main when paired with grains like wild rice or quinoa.
Ingredients
For the Casserole:
- 2 tablespoons unsalted butter
- 1 tablespoon olive oil
- 2 large onions, thinly sliced
- 1/2 teaspoon salt
- 1/4 teaspoon black pepper
- 1 teaspoon sugar
- 3 cloves garlic, minced
- 1 pound cremini or button mushrooms, sliced
- 1/2 teaspoon dried thyme
- 1 tablespoon Worcestershire sauce
- 1/2 cup sour cream
- 1/4 cup mayonnaise
- 1/2 cup shredded Gruyère or Swiss cheese
- 1/2 cup shredded mozzarella cheese
For the Topping:
- 1 cup crushed buttery crackers (like Ritz)
- 1 tablespoon melted butter
- chopped parsley for garnish (optional)
Instructions
- Preheat and Prepare: Preheat your oven to 375°F (190°C) and grease a medium casserole dish to prevent sticking.
- Caramelize Onions: In a large skillet, melt the butter and olive oil over medium heat. Add the thinly sliced onions, salt, pepper, and sugar. Cook for 15–20 minutes, stirring occasionally, until the onions become soft and develop a deep golden color, indicating they are perfectly caramelized.
- Sauté Mushrooms and Garlic: Add the minced garlic and sliced mushrooms to the skillet with the caramelized onions. Continue cooking for another 8–10 minutes until the mushrooms are browned and most of their moisture has evaporated. Stir in the dried thyme and Worcestershire sauce, then remove the skillet from heat.
- Mix the Creamy Base: In a large bowl, combine the sour cream, mayonnaise, shredded Gruyère (or Swiss), and shredded mozzarella cheeses. Stir until blended smoothly.
- Combine Mixture: Add the cooked onion and mushroom mixture to the creamy base and stir everything together until thoroughly combined.
- Assemble the Casserole: Transfer the mixture into the greased casserole dish, spreading it evenly. In a small bowl, mix the crushed buttery crackers with the melted butter, then sprinkle this topping evenly over the casserole.
- Bake: Place the casserole dish in the oven and bake for 20–25 minutes, or until the topping is golden brown and bubbly.
- Garnish and Serve: Remove from the oven, garnish with chopped parsley if desired, and serve warm for a comforting and delicious meal or side dish.
Notes
- This casserole makes a great holiday side dish or a satisfying vegetarian main course.
- For a heartier meal, add cooked wild rice or quinoa to the mixture before baking.
- Leftovers can be stored in the refrigerator and reheated the next day without losing flavor.
- Prep Time: 20 minutes
- Cook Time: 25 minutes
- Category: Side Dish
- Method: Baking, Sautéing
- Cuisine: American
Nutrition
- Serving Size: 1 scoop (about 1/6 of casserole)
- Calories: 320
- Sugar: 6g
- Sodium: 420mg
- Fat: 24g
- Saturated Fat: 10g
- Unsaturated Fat: 12g
- Trans Fat: 0g
- Carbohydrates: 18g
- Fiber: 2g
- Protein: 7g
- Cholesterol: 45mg
