French Onion Soup Recipe

Prepare onions

Peel and halve the onions, then slice each half into thin half-moons (about 1/8-inch thick). Aim for uniform slices so they caramelize evenly.

In a large, heavy-bottomed Dutch oven or pot over medium heat, melt the butter with the olive oil. When butter foams and oil shimmers (about 1 minute), add the sliced onions, spread them into an even layer and sprinkle with the granulated sugar, 1 tsp of the kosher salt, and the black pepper.

Cook the onions over medium to medium-low heat, stirring every 3-5 minutes. Maintain a gentle but steady simmer—not a rapid boil. After about 20-25 minutes the onions will wilt and begin to brown. Continue cooking and stirring more frequently as they deepen in color to avoid burning. Total caramelization time should be 35-45 minutes until they are richly golden-brown and sweet. If the pan seems too dry at any point, add a tablespoon of water to deglaze the fond; do not increase heat to speed up coloring.

Once onions are deeply caramelized, increase heat to medium-high. Pour in the white wine to deglaze the pan, scraping up the browned bits from the bottom with a wooden spoon. Let the wine reduce by about half (2-3 minutes) so the alcohol cooks off and concentrated flavor remains.

Add the beef stock and optional chicken stock to the pot along with the bay leaves and thyme sprigs. Stir to combine and bring to a gentle simmer. Taste and add the remaining 1/2 tsp kosher salt if needed.

Reduce heat to low and simmer uncovered for 20-25 minutes to allow flavors to meld. Skim any foam if it appears. Remove and discard the bay leaves and thyme stems (if using sprigs, you can leave thyme leaves if they’ve fallen off). Stir in the sherry vinegar and Worcestershire sauce, then adjust seasoning with salt and pepper to taste.

While the soup simmers, preheat the oven to 400°F (200°C). Arrange baguette slices on a baking sheet. Brush lightly with 1 tsp olive oil and toast in the oven until golden and crisp, about 6-8 minutes, flipping halfway. Alternatively, broil briefly but watch closely to avoid burning.

As soon as the toasts are warm, rub one side of each slice with the halved garlic clove for a hint of garlic flavor.

Mix the grated Gruyère with the Parmesan (if using) in a bowl. Pile a generous handful of cheese on each toast slice so it will melt well over the soup.

If you prefer to broil assembled bowls: ladle hot soup into oven-safe bowls (leave an inch from rim), place one or two cheese-topped toasts on each bowl so the toast sits on the soup (or place toast on top and sprinkle extra cheese). Put the bowls on a sheet pan and broil on high until cheese is bubbly and golden, about 2-4 minutes. Rotate pan as needed for even browning. Alternatively, place toasted, cheese-topped slices under broiler on a sheet pan and when cheese is melted, transfer to soup bowls.

Serve immediately while very hot. Spoon any remaining cheese-melt over the top, sprinkle a little extra freshly ground pepper, and garnish with a small thyme sprig if desired.

Leftovers: Refrigerate soup (without cheese toast) in an airtight container up to 3 days. Reheat gently on the stovetop and prepare fresh toasts when serving.