French Onion Soup Casserole

This French Onion Soup Casserole takes everything you love about the classic bistro soup – those deeply caramelized onions, rich broth, and gooey melted cheese – and transforms it into a hearty, spoonable casserole that’ll have everyone asking for seconds.

What makes this dish truly special is the double layer of crusty baguette slices and perfectly melted Gruyere cheese that creates an irresistible golden-brown crust. Underneath, you’ll find a rich, savory stew packed with caramelized onions, tender vegetables, and herbs that’ll make your kitchen smell like a Parisian café.

Ingredients

For the stew:

• 8 tablespoons (1 stick) unsalted butter, divided
• 7 large yellow onions (5 to 5 1/2 pounds total), halved and cut into 1/4-inch-thick slices
• 4 large shallots (about 8 ounces), peeled and cut into small dice
• 4 teaspoons kosher salt, divided
• 1 teaspoon freshly ground black pepper, divided
• 6 cloves garlic, minced
• 5 stalks celery (about 12 ounces), including leaves, cut into small dice
• 5 carrots (about 8 ounces), peeled and cut into small dice
• 6 cups cremini and/or button mushrooms (about 1 1/2 pounds), sliced thinly
• Cooking spray
• 1/4 cup all-purpose flour
• 1 cup dry Riesling wine
• 6 to 8 fresh sage leaves, minced, or 2 teaspoons dried sage
• Leaves of 5 sprigs fresh thyme, finely chopped
• 1 (15-ounce) can white beans and their liquid (optional)

For the topping:

• 1 large baguette (about 1 pound), cut crosswise into 24 slices
• 12 ounces Gruyere, Comté, or Raclette cheese, shredded
• 4 tablespoons unsalted butter

Steps

1. Start by melting 4 tablespoons butter in a large Dutch oven over medium-high heat until foamy. This will be the foundation of your flavor base.
2. Add onions and shallots with 1 teaspoon each salt and pepper. Cook for 5-7 minutes until translucent with golden edges. Reduce to low heat and caramelize for 30 minutes, stirring every 5-7 minutes. They should be deep caramel brown – this is where your patience pays off!
3. Incorporate garlic, celery, carrots, and mushrooms in batches. Cover and cook for 20 minutes until vegetables are tender but not mushy. The mushrooms will reduce significantly.
4. Position rack in middle and preheat oven to 375°F (190°C). Coat a 9×13-inch baking dish with cooking spray.
5. Transfer vegetables to a bowl using a slotted spoon. Save that precious cooking liquid – it’s liquid gold!
6. Return pot to high heat, add remaining 4 tablespoons butter until foamy. Add flour, whisking constantly for 2 minutes until golden-blonde. This roux will thicken your sauce perfectly.
7. Pour in wine, whisking to prevent lumps. Add reserved liquid, herbs, and remaining salt. Combine with vegetables and beans (if using).
8. For the topping, layer 12 bread slices, press gently into liquid. Top with half the cheese. Repeat layers, finishing with remaining cheese. Drizzle with melted butter.
9. Bake for 20-22 minutes until bubbly with golden-brown cheese. Let rest 5 minutes before serving – it’ll be molten hot!

Smart Swaps

• Use sourdough instead of baguette for extra flavor depth
• Substitute vegetable stock for wine to make it alcohol-free
• Try Swiss or Emmental cheese if Gruyere isn’t available

Make It Diabetes-Friendly

• Use whole grain bread (reduces carbs by 30%)
• Skip optional beans to lower carb count
• Portion into smaller servings (6 instead of 8)
• Serve with a large green salad to help regulate blood sugar

Pro Tips

• Slice onions uniformly for even caramelization
• Don’t rush the onions – slow cooking develops best flavor
• Use room temperature cheese for better melting
• Let casserole rest 5 minutes before serving to set