Classic French Onion Soup with Gruyère Toast

French Onion Soup

French onion soup gets its sweetness from onions cooked until they are deeply golden and dramatically reduced. That takes patience: the onions need to brown before the beef broth goes in, but they should never scorch. The finished soup should be glossy and brothy, with soft onion ribbons visible in every bowl.

Each serving gets one thick baguette toast topped with melted Gruyère. Broiling the cheese toasts separately gives them lightly browned tops and lets you serve the soup in ordinary bowls without putting them under the broiler.

Ingredients for French Onion Soup

• 3 pounds yellow onions, about 6 large, halved and thinly sliced (1,350g)
• 4 tablespoons unsalted butter (55g)
• 1 tablespoon olive oil
• 1 teaspoon kosher salt, plus more to taste
• 1/2 teaspoon freshly ground black pepper, plus more to taste
• 1/4 teaspoon granulated sugar
• 2 tablespoons all-purpose flour (15g)
• 1/2 cup dry white wine
• 6 cups low-sodium beef broth, plus up to 1 cup more if needed
• 3 fresh thyme sprigs
• 1 bay leaf
• 8 thick baguette slices, about 1/2 inch thick (about 220g); 4 for the bowls and 4 extras
• 8 ounces shredded Gruyère cheese (225g)

Yield: 4 servings. Prep: 15 minutes. Cook: About 1 hour 10 minutes to 1 hour 25 minutes. Total: About 1 hour 25 minutes to 1 hour 40 minutes.

How to Make French Onion Soup

1. Start the onions: Slice the onions thinly from pole to pole to make long ribbons. Melt the butter with the olive oil in a wide, 5- to 6-quart Dutch oven or heavy-bottomed pot over medium heat. Add the onions, salt, pepper, and sugar. Cook uncovered, stirring regularly, until the onions soften.
2. Caramelize the onions: Reduce the heat to medium-low or low. Continue cooking uncovered until the onions have collapsed, turned deep golden brown, and smell sweet rather than sharp, about 45 to 60 minutes total from when they went into the pot. Begin checking their color around 35 minutes. Once softened, stir and scrape the bottom every 3 to 5 minutes. If browned bits begin to stick or darken toward black, lower the heat and stir in 1 to 2 tablespoons of water or broth to loosen them.
3. Add the flour and wine: Stir the flour into the onions until no dry flour remains; cook briefly. Pour in the wine and scrape up the browned bits from the bottom of the pot. Let the wine reduce for 1 to 2 minutes, until its raw aroma fades.
4. Simmer the soup: Add 6 cups of beef broth, the thyme, and the bay leaf. Bring to a simmer over medium heat, then reduce to medium-low. Cook at a gentle bubble for 20 minutes, partially covered or uncovered. Remove the thyme stems and bay leaf. The soup should be glossy and pourable, with broth surrounding the onions. If it has reduced too much, add hot broth a little at a time. Taste before adding more salt or pepper.
5. Toast the bread: While the soup simmers, arrange the baguette slices on a sheet pan and toast in a 375°F (190°C) oven for 6 to 10 minutes, turning once if needed, until dry on the surface and lightly golden at the edges. During the final 10 minutes of simmering, switch the oven to the high broiler and position a rack about 6 inches from the element.
6. Broil the cheese toasts: Put 4 toasted slices on a foil-lined rimmed sheet pan and divide the Gruyère among them. Broil for 1 to 3 minutes, watching continuously from the first minute. Remove the toasts when the cheese is melted and bubbling with a few light golden spots, before it turns dark.
7. Serve: Let the soup stand off the heat for 3 to 5 minutes while the toasts finish. Ladle it into 4 wide bowls and set one cheese toast in the center of each. Leave a ring of broth and onion ribbons visible around the toast, and serve immediately. Offer the extra toasted bread on the side.

How to Caramelize Onions Without Burning Them

Use color and aroma as the final test, not just the clock. Onion moisture, pot width, and burner strength affect how quickly the onions brown. They should be deeply golden and soft enough to collapse into the spoon, without black patches or an acrid smell. Lower the heat as needed, and loosen stuck browned bits with a small splash of water or broth before they scorch.

If the finished soup seems too concentrated or onion-heavy, stir in hot beef broth until liquid flows easily around the onions. Taste again after adding broth.

Why Broil the Cheese Toast Separately?

Toasting the bread before adding cheese gives it a sturdy surface and browned edges. Broil the cheese-topped slices on a sheet pan, then place one on each bowl at the last moment. The lower edge will absorb some broth, while the crust stays visible.

To broil bread and cheese directly on the soup instead, use only crocks confirmed to be broiler-safe and place them on a sheet pan. Do not put glass or unverified bowls under the broiler.

Bread and Cheese Choices

Gruyère gives the topping a nutty flavor and a pale, bubbly melt. Swiss or Emmentaler also melt well, though the flavor will differ. Thick slices of French bread or sturdy sourdough can replace baguette; toast them until the surface feels dry so they do not soften immediately on the soup.

For an alcohol-free version, deglaze with an additional 1/2 cup beef broth instead of wine. The soup will be less bright. For a clearer broth, omit the flour.

Make-Ahead, Storage, and Reheating

Make the soup base up to 2 days ahead, keeping the bread and cheese separate. Cool the soup promptly and refrigerate it in an airtight container for up to 3 to 4 days. Reheat in a saucepan over medium-low heat until steaming, adding a splash of broth if needed. Make the cheese toasts just before serving.

Freeze the cooled soup base without bread or cheese for up to 2 months. Thaw overnight in the refrigerator, reheat gently, and finish with fresh cheese toasts.

Nutrition

Approximate per serving, based on 4 bowls with one cheese toast each and excluding the 4 extra bread slices: 575 calories, 28g protein, 34g fat, and 53g carbohydrates. Approximate per 100g of finished soup with topping: 95 calories, 5g protein, 6g fat, and 9g carbohydrates. Values vary with the broth, bread, cheese, and amount of liquid evaporated.