How to make French onion soup and other cheese-topped soups — the melty, golden cheese crust, the best cheeses, and the technique.
A bubbling, golden cheese crust transforms a bowl of soup into something special — none more famously than French onion soup, with its molten cap of cheese over a crouton. Here's how to make French onion soup and other cheese-topped soups, with that irresistible gratinéed top.
The Glory of French Onion Soup
French onion soup (soupe à l'oignon gratinée) is the classic cheese-topped soup: deeply caramelized onions in a rich beef broth, topped with a slice of toasted bread and a thick layer of cheese, then grilled until the cheese is melted, bubbling, and golden. The contrast of savory broth, sweet onions, soggy-crisp crouton, and stretchy melted cheese makes it one of the most beloved soups in the world.
Caramelizing the Onions
The foundation of French onion soup is properly caramelized onions, and this takes patience. Cook sliced onions slowly in butter over low-medium heat for a long time — often 45 minutes or more — stirring, until they turn deep golden brown, sweet, and jammy. Rushing this step (or settling for pale onions) gives a thin, bland soup. The deep caramelization is what gives the broth its rich, sweet, savory depth. A splash of wine or sherry deglazes the pan and adds complexity.
Choosing the Cheese
The classic cheese for French onion soup is Gruyère, which melts smoothly into a nutty, savory, golden crust. Comté, Emmental, or a Swiss-style cheese also work well, and a little Parmesan on top helps it brown. The cheese needs to melt beautifully and brown under the broiler — Gruyère does both perfectly. Use plenty, grated or sliced, so it forms a generous, bubbling cap.
The Gratinéed Top
The signature of French onion soup is the gratinéed cheese top. Ladle the soup into ovenproof bowls, float a slice of toasted baguette (the crouton) on top, pile generously with grated Gruyère, and place under a hot broiler (or in a very hot oven) until the cheese is melted, bubbling, and golden brown, often dripping over the edges of the bowl. This melty, slightly crisp cheese crust is what everyone loves. Use heatproof bowls and handle them carefully, as they get very hot.
Other Cheese-Topped Soups
The same technique elevates other soups. A cheese crust works beautifully on tomato soup, French onion's cousins, and hearty vegetable or bean soups. Beer-cheese soup and broccoli-cheddar soup incorporate cheese into the soup itself, while topping any rich soup with a cheesy crouton and broiling it adds indulgence. Gruyère, cheddar, and Parmesan are all great choices depending on the soup's flavor.
Serving
Serve cheese-topped soups immediately, while the cheese is molten and the crust is golden, with care since the bowls are hot from the broiler. The pleasure is in breaking through the cheese cap into the soup below. A simple salad and more bread make it a satisfying meal. French onion soup, especially, is a cozy, restaurant-worthy dish you can make at home.
Frequently Asked Questions
What cheese is best for French onion soup?
Gruyère is the classic, melting into a nutty, golden crust; Comté, Emmental, or Swiss-style cheese also work, often with a little Parmesan.
Why caramelize the onions so long?
Slow, deep caramelization gives the soup its rich, sweet, savory flavor — pale or rushed onions make a thin, bland broth.
How do I get the bubbling cheese top?
Top the soup with a toasted bread slice and plenty of grated cheese, then broil in heatproof bowls until melted, bubbling, and golden.