If you’re on the hunt for a comforting, crowd-pleasing dish that’s both elegant and approachable, look no further than this French Onion Soup Casserole. With all of the rich, savory flavors of the classic soup—in layers of caramelized onions, toasty bread, and gooey cheese—this casserole transforms a bistro favorite into the ultimate family meal. Every bite has just the right balance of sweet onions, fragrant thyme, and that irresistible blanket of melted Gruyère. Whether you’re serving it for a cozy weeknight dinner or wowing friends at a gathering, this French Onion Soup Casserole delivers all the magic of the original, but in a format that’s easy, satisfying, and guaranteed to impress.

French Onion Soup Casserole Recipe - Recipe Image

Ingredients You’ll Need

French Onion Soup Casserole comes together with just a handful of simple, everyday ingredients, but each one plays a starring role in building the irresistible depth and character of the dish. From the sweetness of caramelized onions to the creaminess of melting cheese, every item here brings something special to your table.

Yellow Onions (4 large, thinly sliced): Go for yellow onions—they’re sweet and mellow, perfect for caramelizing into golden perfection.
Unsalted Butter (3 tablespoons): Using butter adds that buttery richness and helps the onions caramelize just right.
Olive Oil (1 tablespoon): A splash of oil prevents the butter from burning and adds lovely flavor.
Sugar (1 teaspoon): Just a bit of sugar encourages the onions to develop that deep, signature sweetness.
Salt (1/2 teaspoon): A pinch of salt draws out moisture and intensifies flavors.
Black Pepper (1/2 teaspoon): Freshly ground pepper adds a gentle kick and rounds out the taste.
Thyme (1 tablespoon fresh or 1 teaspoon dried): Earthy thyme is classic in French onion soup—don’t skip it!
Garlic (2 cloves, minced): Garlic brings just the right amount of aromatic flair to the party.
Dry White Wine (1/4 cup, optional): Adding wine helps deglaze the pan and layers in subtle complexity (feel free to skip or use broth instead!).
All-Purpose Flour (1 tablespoon): A small spoonful thickens the sauce beautifully without being heavy.
Low-Sodium Beef Broth (2 cups): Broth is the backbone of the casserole’s savory, hearty flavor (or use veggie broth for a vegetarian version).
Worcestershire Sauce (1 tablespoon): A dash of Worcestershire brings umami depth you don’t want to miss.
French Bread (6 slices, lightly toasted): Crusty toasted bread soaks up all the deliciousness and adds texture.
Gruyère or Swiss Cheese (2 cups, shredded): You’ve got to have that crown of shredded, melty cheese—Gruyère is traditional, but Swiss works too.

How to Make French Onion Soup Casserole

Step 1: Caramelize the Onions

Start by heating the butter and olive oil in a large skillet over medium heat. Add the sliced onions, sugar, salt, and black pepper. Stir frequently as the onions slowly break down and turn golden brown, about 25 to 30 minutes. This is the most important step: take your time and let the onions get deeply caramelized—this is where the magic (and the sweet, rich flavor) happens!

Step 2: Add Garlic and Herbs

Once the onions are beautifully golden, stir in the thyme and garlic. Let them cook for a minute, just until fragrant. This step infuses the base with an herbal, garlicky note that elevates the whole casserole.

Step 3: Deglaze and Make the Sauce

If you’re using wine, pour it into the pan and scrape up any flavorful browned bits on the bottom—those bits add serious flavor! Give the mixture a minute or two to cook off the alcohol. Next, sprinkle in the flour and stir for 1–2 minutes; this will thicken your sauce just enough. Gradually add the beef broth and Worcestershire sauce, stirring constantly. Let it all simmer for five minutes until slightly thickened and irresistibly fragrant, then remove from heat.

Step 4: Layer the Casserole

Now it’s time for the fun part: assembling your French Onion Soup Casserole! Grease a 9×13-inch dish and spread half of the onion mixture on the bottom. Lay three slices of toasted French bread on top, then sprinkle over half of your shredded cheese. Repeat with the remaining onions, bread, and finally, the rest of the cheese. Think of it like building a cheesy, savory lasagna—but way easier.

Step 5: Bake and Broil

Pop the casserole into your preheated oven and bake uncovered for 20–25 minutes, until the cheese is melted, bubbling, and calling your name. For an extra golden top, broil for the last 2–3 minutes. Let your masterpiece rest for a few minutes out of the oven to allow everything to set—patience pays off here!

How to Serve French Onion Soup Casserole

Garnishes

Right before serving, make it a showstopper by showering your French Onion Soup Casserole with a few fresh thyme leaves or a sprinkle of chopped parsley. If you love a sharper bite, try a light twist of freshly cracked black pepper or a dusting of extra Gruyère. A drizzle of good olive oil or even a quick brush of garlic butter on top of the bread slices also adds a luscious finish.

Side Dishes

This casserole pairs up perfectly with a crisp green salad tossed in a tangy vinaigrette, or some simply roasted vegetables to balance its richness. For a heartier meal, serve it alongside a roasted chicken or a platter of grilled sausages. Even a simple bowl of mixed olives or pickled vegetables can really set off the savory flavors in your French Onion Soup Casserole.

Creative Ways to Present

If you want to wow your guests, bake the casserole in individual oven-safe bowls for a bistro-style presentation. For a fun, rustic touch, use a cast-iron skillet for baking and serve straight from the pan. You could even slice leftover casserole into squares and serve it as a surprisingly crowd-pleasing appetizer at your next gathering!

Make Ahead and Storage

Storing Leftovers

Once cooled, keep any leftover French Onion Soup Casserole in an airtight container in the fridge. It holds up beautifully for up to three days, and the flavors only deepen as it sits—if it lasts that long!

Freezing

Yes, you can freeze French Onion Soup Casserole! Wrap the cooled casserole tightly in two layers of foil or transfer to a freezer-safe dish. It will be good for up to two months. When you’re ready to enjoy, thaw it overnight in the refrigerator before reheating.

Reheating

To reheat, cover the dish with foil and warm it in a 350°F oven until piping hot and bubbly, about 20 minutes. For a crispier top, remove the foil for the last few minutes. You can also microwave slices individually, but the oven does the best job of reviving the gorgeous cheesy texture.

FAQs

Can I make this casserole vegetarian?

Absolutely! Just switch out the beef broth for your favorite vegetable broth. You’ll get all the wonderful French Onion Soup Casserole flavors with a lovely, plant-based twist—just be sure to check your Worcestershire sauce, as some brands are not vegetarian.

Is there a substitute for Gruyère cheese?

Yes! While Gruyère is the traditional choice for French Onion Soup Casserole thanks to its nutty, meltable qualities, Swiss cheese, mozzarella, or even provolone work well in a pinch. Mix and match to suit your taste (or what you have on hand)!

What kind of bread works best?

Day-old French bread is ideal, as it’s sturdy enough to absorb all the savory broth without falling apart. You can also use a rustic baguette, ciabatta, or even sourdough for a different twist and texture.

How can I make sure my onions caramelize properly?

Patience is key! Keep the heat at medium, stir often, and resist the urge to crank up the heat. The onions will gradually turn deep golden and sweet—don’t rush this step, as it’s where all the signature French Onion Soup Casserole flavor develops.

Can I prep this casserole ahead of time?

Definitely. Assemble the casserole (hold off on baking), cover, and refrigerate for up to a day in advance. Pop it in the oven when you’re ready—perfect for entertaining or busy nights!

Final Thoughts

There’s something magical about sharing warm, bubbling French Onion Soup Casserole from your oven—a savory hug in casserole form! I hope you love making (and eating) this recipe as much as I do. Give it a try, and don’t forget to share your delicious results with friends and family—this dish is meant to be enjoyed together, and it never fails to impress!

French Onion Soup Casserole Recipe

5 Stars 4 Stars 3 Stars 2 Stars 1 Star

4.6 from 8 reviews

Author: Emma
Total Time: 1 hour 10 minutes
Yield: 6 servings 1x
Diet: Non-Vegetarian

Print Recipe

Description

Indulge in the rich flavors of French Onion Soup with this comforting casserole. Caramelized onions, savory broth, toasty bread, and gooey cheese come together in a hearty dish perfect for chilly nights.

Ingredients

Scale

Onion Mixture:

4 large yellow onions, thinly sliced
3 tablespoons unsalted butter
1 tablespoon olive oil
1 teaspoon sugar
1/2 teaspoon salt
1/2 teaspoon black pepper
1 tablespoon fresh thyme leaves or 1 teaspoon dried thyme
2 cloves garlic, minced
1/4 cup dry white wine (optional)
1 tablespoon all-purpose flour
2 cups low-sodium beef broth
1 tablespoon Worcestershire sauce

Assembly:

6 slices French bread, lightly toasted
2 cups shredded Gruyère cheese or Swiss cheese

Cook Mode Prevent your screen from going dark

Instructions

Preheat and Caramelize Onions: Preheat oven to 375°F (190°C). Cook onions with butter, olive oil, sugar, salt, and pepper until caramelized. Add thyme, garlic, and white wine.
Make Sauce and Layer: Stir in flour, then add beef broth and Worcestershire sauce. Layer onion mixture, bread, and cheese in a casserole dish.
Bake: Bake for 20-25 minutes until cheese is bubbly. Broil for a golden top if desired. Rest before serving.