A rich, comforting twist on the classic French onion soup—this casserole layers deeply caramelized onions, savory broths, toasted bread, and two kinds of melty cheese into a golden, bubbling bake. With all the deep umami flavor of the original soup but in hearty, sliceable form, it’s perfect for cozy dinners, holiday gatherings, or anytime you want restaurant-quality comfort at home.
Why You’ll Love This Recipe
This isn’t just another casserole—it’s elevated comfort food. Inspired by the beloved French onion soup, this version transforms tender sweet onions, wine, and broth into a thick, flavorful base, then builds it up with crunchy toasted bread and a blanket of Gruyère and Parmesan. Baked until golden and bubbly, every bite delivers savory depth, cheesy richness, and that irresistible crisp-tender texture.
Perfect for:
Fall and winter meals
Holiday entertaining
Make-ahead dinners
Feeding a crowd without last-minute stress
🧅 “I served this at my Thanksgiving open house,” said one home cook. “People thought I’d been cooking for hours—but most of it happened while I prepped other dishes!”
Ingredients
For the Caramelized Base
3 tbsp (42g) unsalted butter
3 large sweet onions, thinly sliced (~6 cups) (Vidalia or Walla Walla work great)
2 cloves garlic, minced
Salt and freshly ground black pepper, to taste
1 tsp fresh or dried thyme (use ½ tsp if dried)
For the Broth & Roux
2 tbsp (16g) all-purpose flour
½ cup (120ml) dry white wine (like Sauvignon Blanc or Pinot Grigio)
2 cups (480ml) beef broth (low-sodium recommended)
1 cup (240ml) chicken broth (or all beef broth for deeper flavor)
For Layering & Topping
6 slices day-old French bread or baguette, toasted until golden
1½ cups (6 oz / 170g) Gruyère cheese, grated (essential for authentic flavor!)
½ cup (25g) Parmesan cheese, freshly grated
¼ cup (10g) fresh parsley, chopped (for garnish)
For Prep
Cooking spray or butter, for greasing the baking dish
Step-by-Step Instructions
1. Preheat & Prep
Preheat oven to 400°F (200°C).
Lightly grease a 9×13-inch baking dish with cooking spray or butter.
2. Caramelize the Onions
In a large Dutch oven or heavy skillet over medium heat:
Melt butter. Add sliced onions and stir to coat.
Cook 12–15 minutes, stirring occasionally, until soft, golden brown, and sweet.
✅ Low and slow wins —don’t rush; browning builds flavor.
3. Add Aromatics
Stir in minced garlic, thyme, salt, and pepper.
Cook 1 minute more, until fragrant.
4. Make the Roux & Deglaze
Sprinkle flour over onions and stir constantly 2 minutes to cook out raw taste.
Pour in white wine, scraping up any browned bits from the bottom of the pan.
Simmer 3–4 minutes, until wine is reduced by half and mixture is thick.
5. Simmer the Broth
Gradually whisk in beef broth and chicken broth.
Bring to a gentle simmer and cook 10 minutes, stirring occasionally, to deepen flavors.
Remove from heat and taste—adjust seasoning as needed.
6. Assemble the Layers
In the prepared baking dish, layer in this order:
Half of the onion-broth mixture
Half of the toasted bread slices
Half of the Gruyère + Parmesan cheeses
Repeat the layers:
4. Remaining onion mixture
5. Remaining bread
6. Remaining cheeses
✅ Press down lightly —helps everything soak in evenly.
7. Bake Until Golden & Bubbly
Bake uncovered for 20–25 minutes, until:
Cheese is melted, bubbly, and lightly browned on top
Edges are gently sizzling
✅ No need to cover —you want that crisp, golden crust!
8. Rest & Serve
Let rest 5 minutes before serving—this allows layers to set slightly for cleaner scooping.
Garnish with chopped fresh parsley.
Serve warm as a main course or hearty side.
✅ Optional upgrade: Serve with a green salad or roasted vegetables.
You Must Know
Slice onions evenly —ensures even cooking and caramelization.
Don’t skip deglazing —those browned bits are flavor gold.
Use real Gruyère —it melts beautifully and adds nutty depth.
Toast stale bread well —crisp slices hold up better than soggy ones.
Let it rest —hot casseroles fall apart when scooped too soon.