French Onion Soup Recipe

Prepare onions

Peel and halve the onions from root to tip, then thinly slice across the grain to yield about 6 cups. Keep slices uniform for even caramelization.

Caramelize onions

In a large heavy-bottomed pot or Dutch oven, melt the butter with 1 tablespoon olive oil over medium heat. Add the sliced onions, sprinkle with 1 teaspoon sugar and 1 teaspoon fine sea salt. Cook, stirring every 3–5 minutes, until onions become soft and translucent (about 15–20 minutes). Reduce heat to medium-low and continue to cook, stirring more frequently and scraping browned bits from the bottom, until deep golden brown and richly caramelized, about 30–40 more minutes. If onions start to stick or char, add a splash (1–2 tbs) of water to deglaze and continue.

Deglaze & thicken

Increase heat to medium-high. Add the dry white wine to the pot to deglaze, scraping up browned fond from the bottom. Simmer until wine is reduced by half (about 3–4 minutes). Sprinkle 2 tablespoons flour over the onions, stir to coat and cook 1–2 minutes to remove raw flour taste.

Simmer soup

Slowly whisk or stir in 6 cups beef broth until smooth. Add 1 tablespoon beef demi-glace (or concentrated stock), 3 thyme sprigs, 2 bay leaves, and 1 teaspoon Worcestershire sauce. Bring to a gentle simmer, reduce heat to low and cook uncovered for 20–25 minutes to meld flavors. Taste and adjust seasoning with salt and freshly ground black pepper.

Finish

Remove and discard thyme stems and bay leaves. Keep soup hot while you prepare crostini and cheese topping.

Prepare crostini

Preheat oven to 400°F (200°C). Arrange baguette slices on a baking sheet. Brush lightly with 2 teaspoons olive oil. Toast in the oven until edges are golden, about 6–8 minutes, flipping once for even browning. Remove and rub each warm slice with the cut side of the garlic clove.

Assemble bowls

Preheat broiler. Ladle hot soup into ovenproof bowls or crocks, leaving about 1/2 inch headspace. Place one or two toasted crostini on top of each bowl of soup (or under, if you prefer fully submerged cheese). Generously mound grated Gruyère over the crostini and soup surface. Sprinkle a little grated Parmesan over each if using.

Melt cheese

Place bowls on a baking sheet and broil 4–6 inches from heat until cheese is melted, bubbly and lightly browned, about 2–4 minutes. Watch carefully to prevent burning. Alternatively, bake at 425°F (220°C) for 8–10 minutes until cheese is melted.

Serve

Carefully remove hot bowls from oven (use oven mitts). Let sit 1–2 minutes, then garnish with a small thyme sprig if desired. Serve immediately.