French Onion Soup Recipe

Prepare onions

Peel and halve the onions, then thinly slice them crosswise so you have about 6 cups of sliced onions.

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

Add the sliced onions, spread them evenly in the pot, and cook over medium to medium-low heat, stirring every few minutes. After about 10 minutes, sprinkle the sugar and kosher salt over the onions to encourage caramelization.

Continue to cook the onions slowly, stirring more frequently as they start to brown, for 35–45 minutes total. Adjust heat as needed to avoid burning—you want deep golden-brown color and sweet aroma. If browned bits form on the bottom, scrape them up with a wooden spoon.

In the last 2 minutes of caramelizing, add the minced garlic and freshly ground black pepper and cook until fragrant, about 1–2 minutes.

Increase the heat to medium-high and add the white wine (or vermouth) to deglaze the pot. Use a wooden spoon to scrape up any browned bits from the bottom. Let the wine reduce by about half, about 2–3 minutes.

Add the beef stock and chicken stock (if using) to the pot. Stir in the Worcestershire sauce, bay leaves, and thyme sprigs.

Bring the soup to a gentle simmer, then reduce heat to maintain a low simmer. Cook uncovered for 20–30 minutes to let flavors meld and reduce slightly. Taste and adjust seasoning with salt and more pepper if needed. Remove and discard the bay leaves and thyme sprigs (or strip leaves off and return them if preferred).

While the soup simmers, preheat your oven broiler. Arrange baguette slices on a baking sheet and toast under the broiler until golden on both sides, about 1–2 minutes per side. Watch carefully to avoid burning.

Ladle the hot soup into oven-safe bowls or crocks, leaving about 1/2-inch headspace. Place 1–2 toasted baguette slices on top of each bowl of soup (you can break slices to fit).

Evenly top each baguette-covered bowl with a generous layer of grated Gruyère (about 1/4 cup per serving) and a light sprinkle of grated Parmesan if using.

Place the bowls on a baking sheet and broil in the oven until the cheese is melted, bubbly, and lightly browned, 2–4 minutes depending on broiler strength. Alternatively, use a kitchen torch to brown the cheese.

Carefully remove from oven (use oven mitts) and let cool 2–3 minutes. Garnish with a pinch of fresh thyme leaves and serve hot.