Few dishes embody cozy indulgence like a French Onion Mushroom Casserole. A savory, bubbling bake that marries the sweetness of slow-cooked onions with the umami earthiness of mushrooms and the rich decadence of a creamy sauce—this casserole is a celebration of comforting, meatless goodness.
Whether you’re crafting a weekend meal, preparing a dish for a holiday potluck, or simply exploring vegetable casserole ideas that go beyond the basics, this recipe will become a repeat favorite in your kitchen. It’s an ode to French onion soup but reborn as a hearty casserole. Add in crispy French bread cubes or French fried onions, and you’ve got texture, flavor, and satisfaction in every bite.
🍄 The Inspiration Behind the Dish
The concept of combining French onion flavors with a creamy mushroom base brings together two deeply flavorful traditions. French onion soup, often topped with broiled bread and gooey cheese, is iconic. Combine that with the richness of a cream of mushroom casserole and the result is something irresistibly decadent yet easy to make vegetarian.
It’s the kind of dish that speaks to all seasons – light enough for spring, hearty enough for winter, and nostalgic enough to serve at any gathering. It’s also a wonderful canvas for layering other vegetables like artichokes, courgettesou même cauliflower. Plus, it stores well, making it ideal for meal prep or leftovers the next day.
🛒 Ingredients – What You’ll Need
Here’s a closer look at everything you need to create this beautiful vegetarian casserole:
✨ Base Ingredients
- 2 cuillères à soupe d'huile d'olive – for sautéing and adding richness.
- 2 cuillères à soupe de beurre – enhances caramelization.
- 4 large yellow onions, thinly sliced – these caramelize down to a sweet, golden base.
- 16 oz cremini or button mushrooms, sliced – earthy and savory, they soak up flavor beautifully.
- 3 garlic cloves, minced – adds a pungent note to balance the sweetness.
- 1 cuillère à café de sel et 1/2 cuillère à café de poivre noir – basic seasoning.
- 1 cuillère à café de thym séché – classic herb for both onions and mushrooms.
- 1/2 cup dry white wine or vegetable broth – deglazes the pan and adds flavor depth.
✨ Creamy Sauce
- 2 cuillères à soupe de beurre
- 2 tablespoons all-purpose flour
- 1½ cups vegetable broth
- 1 cup whole milk or unsweetened almond/oat milk
- ½ cup sour cream or plain Greek yogurt
- ½ cup shredded mozzarella cheese
- ¼ cup grated Parmesan cheese
✨ Topping Options
- 2 cups cubed French bread, tossed in olive oil and herbs – the classic, rustic touch.
- 1 cuillère à soupe d'huile d'olive
- ½ cup shredded Gruyère or Swiss cheese – for melty goodness.
- Optional: ½ cup French fried onions – for added crunch and nostalgic flavor.
🔪 Step-by-Step Instructions
1. Caramelize the Onions
This step is the heart of the dish. Melt butter and olive oil in a skillet over medium-low heat. Add sliced onions and stir to coat. Cook gently, stirring every few minutes, for 25–30 minutes until the onions are deeply golden and fragrant.
💡 TIP: Don’t rush! Low and slow caramelization brings out the natural sweetness of the onions and is key to that signature French onion flavor.
2. Sauté the Mushrooms
Once the onions are caramelized, add the mushrooms and increase heat slightly. Let them cook for about 8–10 minutes, until they release moisture and turn golden. Add minced garlic, thyme, salt, and pepper. Pour in white wine or broth and deglaze the pan, scraping up any browned bits. Simmer until most of the liquid evaporates.
3. Make the Creamy Sauce
In a separate saucepan, melt butter and whisk in the flour to make a roux. Slowly add vegetable broth and milk while whisking. Let the sauce simmer and thicken, about 5 minutes. Remove from heat and stir in sour cream, mozzarella, and Parmesan. Adjust salt and pepper to taste.
4. Assemble the Casserole
Preheat oven to 375°F (190°C). Combine the onion-mushroom mixture with the creamy sauce. Pour into a greased 9×13″ casserole dish.
5. Add the Topping
Top with cubed French bread tossed in olive oil, salt, and herbs. Sprinkle shredded Gruyère on top. For more crunch, sprinkle French fried onions over the cheese layer.
6. Cuire au four
Place in the oven and bake for 25–30 minutes until golden, bubbly, and slightly crispy on top. For an extra crisp top, broil the casserole for 2–3 minutes at the end.
7. Servir
Let the casserole rest for 5–10 minutes before serving. This helps the layers settle and flavors to meld. Garnish with parsley or more thyme if desired.
🧀 Make It Your Own – Variations and Tips
🥦 Add-In Ideas:
- Artichoke hearts (canned or frozen) for a Mediterranean twist.
- Zucchini or spinach pour increase vegetable content.
- Cauliflower florets ou brocoli for a hearty crunch.
🥖 Topping Swaps:
- Use panko breadcrumbs for a light and crispy crust.
- Try gluten-free bread cubes ou sweet potato chips for dietary variations.
- Sprinkle extra French fried onions for a Southern-inspired twist.
🌱 Vegan Options:
- Use vegan butter and plant-based milk.
- Substitute cheeses with vegan mozzarella and nutritional yeast.
- Choose coconut yogurt or a plant-based sour cream alternative.
🍽️ How to Serve French Onion Mushroom Casserole
This casserole is a complete comfort meal on its own, but here are some ideas for turning it into a multi-course spread:
As a Main Course:
- Pair with a crisp arugula or kale salad with lemon vinaigrette.
- Serve with a warm French baguette or garlic toast.
- Add a light roasted tomato soup as a starter.
Comme un Plat d'accompagnement:
- Serve alongside roast chicken, stuffed bell peppersou grilled fish.
- A great pairing for holiday meals like Thanksgiving or Easter.
For Brunch or Breakfast:
- Top with a poached or fried egg.
- Serve with hash browns and fruit salad.
🧊 Storage and Meal Prep
Le ranger:
- Keep leftovers in an airtight container in the fridge for up to 4 days.
- Reheat in the oven at 350°F (175°C) until warmed through.
Congélation:
- Freeze before baking (without the topping) for up to 2 mois.
- Thaw overnight in the refrigerator, add the topping, then bake as directed.
Prenez De L'Avance:
- Caramelize onions and mushrooms a day ahead.
- Assemble casserole and refrigerate up to 24 hours before baking.
🌟 Why This Recipe Works
Cette French Onion Mushroom Casserole shines because it balances multiple elements:
- Flavor: Caramelized onions, mushrooms, and herbs create savory richness.
- Texture: Creamy filling meets a crispy topping for the ultimate mouthfeel.
- Nutrition: Packed with fiber, antioxidants, and plant-based nutrients.
- Comfort: Warm, hearty, nostalgic, and indulgent – without being heavy.
- Versatility: Customizable to fit your dietary needs and pantry staples.
💬 Reader-Favorite Reviews
“This is my go-to weekend comfort food! The onion flavor is amazing and I love the crispy topping. Even my meat-loving husband went back for seconds.” – @vegmealslover
“Tried this with artichokes and vegan cheese – it turned out incredible. Perfect for holidays or just a fancy Tuesday!” – @plantbasedbeth
“The French bread on top made this feel so rustic and homemade. Total 10/10.” – @comfortfoodie
French 
English 
Spanish 
Portuguese