French Onion Soup Recipe

Prepare onions

Peel and halve the onions, then thinly slice each half into 1/8–1/4-inch slices so you have about 6 cups packed sliced onions.

Melt butter and oil

In a large heavy-bottomed pot or Dutch oven, heat 3 tablespoons unsalted butter and 1 tablespoon olive oil over medium heat until the butter foams but does not brown.

Caramelize onions

Add the sliced onions, 1 teaspoon fine sea salt, 1 teaspoon sugar, and 1 teaspoon freshly ground black pepper. Stir to coat, then cook over medium to medium-low heat, stirring every 3–5 minutes. Adjust heat as needed to avoid burning. Continue cooking for 35–45 minutes until onions are deeply golden-brown and sweetly caramelized. If the pan starts to look dry or onions stick, add 1–2 tablespoons of water to deglaze and prevent burning.

Deglaze with sherry

Increase heat to medium-high and pour in 1 cup dry sherry (or dry white wine). Scrape the browned bits from the bottom of the pot with a wooden spoon. Cook 2–3 minutes until most of the alcohol aroma has evaporated and the liquid has reduced slightly.

Add stock and aromatics

Carefully add 6 cups beef broth, 2 tablespoons beef stock concentrate or demi-glace (if using), 3 sprigs fresh thyme, 1 bay leaf, and 1 tablespoon Worcestershire sauce. Stir, bring to a gentle simmer, then reduce heat to low. Simmer uncovered for 20–25 minutes to let flavors meld.

Finish the soup

Remove and discard thyme sprigs and bay leaf. Taste and adjust seasoning with salt and pepper as needed. Stir in 1 teaspoon sherry vinegar if using to brighten the flavor. Keep soup hot while you prepare the toasts and cheese.

Prepare toasts

Preheat oven to 400°F (200°C) or set broiler to high on a rack about 6 inches from the element. Lightly brush both sides of the brioche or baguette slices with the softened butter.

Toast bread

Arrange slices on a baking sheet and toast in the oven 6–8 minutes, flipping once, until golden and lightly crisp. If using broiler, broil 1–2 minutes per side—watch closely to avoid burning.

Top with cheese

Remove toasts from oven. Pile each toast with a generous amount of grated Gruyère (about 1/4 cup per slice). If desired, sprinkle a little grated Parmesan on top.

Broil until bubbly

Set oven to broil and place the cheese-topped toasts on the baking sheet under the broiler until cheese is melted, bubbly, and lightly browned (about 2–3 minutes). Watch carefully.

Assemble soup bowls

Ladle hot soup into oven-safe bowls or crocks, leaving roughly 1/2 inch headspace. Float one or two cheese-toasts on top of each bowl (or place toast on top and sprinkle extra cheese over the soup). Alternatively, place a cheese-topped toast on each bowl and sprinkle a little extra Gruyère.

Final broil (optional)

If you prefer the cheese sealed to the soup, place filled bowls on a rimmed baking sheet and broil until the cheese is blistered and browned, about 2–4 minutes. Remove carefully with oven mitts.

Serve

Garnish with a pinch of fresh thyme leaves and a crack of black pepper. Serve immediately — soup will be very hot.