French Onion Casserole

This French onion casserole recipe takes all the delicious, comforting elements of French onion soup and turns them into a side dish you can put on a plate. Silky caramelized onions are tossed with flour and butter to make a rich sauce, then topped with croutons and layers of melted cheese. It’s perfect next to homey braised beef or casual roast beef sliders.

Ingredients for French Onion Casserole

• Onions: A generous amount of onions are thinly sliced and cooked until very tender. You can pretty much use any type of onion here, but I always use yellow onions. When they caramelize, the yellow onions become sweet but still have a slightly pungent flavor.
• Butter: Cooking onions in butter allows them to soften and turn slightly golden, delivering luxurious flavor.
• Flour: All-purpose flour melts together with butter and broth to thicken the sauce.
• Beef broth: Store-bought or homemade broth brings the flavors of French onion soup together for this casserole.
• Sherry: This fortified wine is a secret ingredient that adds a sweet, nutty complexity to this dish, and it’s also great in other soups like split pea soup or mushroom soup. If you’d prefer not to use any alcohol, you can also use additional beef broth.
• Croutons: Salad croutons are a simple way to top this dish with bread, and they hold up well to the sauce and melted cheese. Use a store-bought variety for ease, or make homemade croutons.
• Swiss cheese: Nutty shredded Swiss cheese melts beautifully. Buy a block of Swiss cheese and shred it fresh for the best flavor and meltability. Gruyere or a young Comté are good substitutes.
• Parmesan cheese: Grated Parmesan cheese doesn’t melt, but it adds a lot of flavor to the cheese topping. You can use Pecorino Romano if you prefer.

Directions

Step 1: Cook the onions and make the sauce

In a large ovenproof skillet (cast-iron is a great choice), melt 3 tablespoons of butter and saute the onions until tender and lightly golden brown. Stir in the flour and pepper until the onions are well coated.

Editor’s Tip: Really good caramelized onions take about 45 minutes, so make sure to build in a bit of time for this step.

Gradually add the broth and sherry. Bring to a boil, then cook, stirring until thickened, about two minutes.

Step 2: Top with croutons and cheese

In a microwave, melt the remaining 2 tablespoons of butter, then add to the croutons in a bowl, tossing to coat.

Distribute the croutons evenly over the onion mixture and top with cheese.

Step 3: Broil the casserole

Broil the casserole 3 to 4 inches from the heat until the cheese is melted and bubbly, one to two minutes.

French Onion Casserole Variations

• Top with fresh herbs: Minced fresh parsley, chives, thyme or a combination of all three would add a bit of fresh green to this rich casserole and complement the onion flavor. Sprinkle them on after broiling.
• Add mushrooms: Sliced mushrooms, sauteed with the onions, would add even more umami depth to this delicious dish.
• Swap in dried bread for the croutons: If you don’t have croutons, toasted baguette slices or cubed dried bread (try a dark rye) would also work. We even like to make Hawaiian roll croutons.

How to Store French Onion Casserole

To store the French onion casserole, allow it to cool to room temperature, then transfer it to an airtight container. Leftover French onion casserole can be stored in the refrigerator for up to four days.

Can you make French onion casserole ahead of time?

To make this casserole ahead of time, cook the onions with the flour and broth. The mixture can be refrigerated for one to two days. When you’re ready to cook, gently reheat the onion mixture in the skillet and top with the prepared croutons and cheese. Broil as directed.

French Onion Casserole Tips

Can I add meat to French onion casserole?

If you’d like to add protein to this dish, cook 1 pound of ground beef first, then drain it and set it aside. Stir it into the onion mixture after cooking, then top as directed. You could also add cooked bacon bits on top after broiling.

How do I make French onion casserole vegetarian or vegan?

If you’d like to make this dish vegetarian, swap in mushroom broth for the beef broth. To make it vegan, swap in olive oil or vegan butter for the butter. Double-check that the croutons are vegan too.

What are the best types of onions to use in French onion casserole?

Yellow onions will cook up sweet, with an onion flavor that is just assertive enough to shine through the croutons and cheese. Although you can use any type of onion here, red onions would create an extremely pungent flavor, whereas white onions or Vidalia onions would lean more sweet than pungent.