French Onion Soup Recipe

Prepare onions

Peel and cut off root ends of the onions. Slice each onion in half pole-to-pole, then slice thinly (about 1/8"–1/4"). You should have about 2 lb thinly sliced onions.

Caramelize onions

In a large, heavy-bottomed pot or Dutch oven, heat 1 tablespoon olive oil and 3 tablespoons unsalted butter over medium heat until butter is melted and foaming. Add all sliced onions, 1 teaspoon sea salt, 1/2 teaspoon freshly ground black pepper, and 1/2 teaspoon granulated sugar. Stir to coat. Cook onions gently, stirring frequently, for 30–45 minutes — reduce heat to medium-low if they start to brown too fast. Scrape browned bits from bottom periodically; the goal is deep golden-brown, sweet onions (do not burn).

Deglaze

When the onions are richly browned, increase heat to medium and add 1/4 cup dry sherry or dry white wine (if using). Scrape the bottom of the pot with a wooden spoon to release any fond; cook until the liquid has mostly evaporated, about 2–3 minutes.

Simmer soup

Add 8 cups beef broth, 1/4 cup beef demi-glace (if using), 3 sprigs fresh thyme, and 2 bay leaves. Bring to a gentle simmer, then reduce heat and simmer uncovered for 20–30 minutes to meld flavors. Taste and adjust seasoning with additional salt and pepper as needed. Remove thyme sprigs and bay leaves before serving.

Prepare bread

Preheat oven to 400°F (200°C). Arrange the baguette slices on a baking sheet. Brush each slice lightly with olive oil on both sides. Toast in the oven until just golden and dry, about 6–8 minutes, flipping once halfway. If you prefer a crisper crostini, toast a little longer. Remove and let cool slightly.

Assemble gratinée

Switch oven to broil (or preheat a kitchen torch). Ladle soup into oven-safe bowls (oufflé dishes or crocks) leaving about 1/2" headspace. Place 1–2 toasted baguette slices on top of each bowl, then divide the grated Gruyère evenly on top of the bread (about 1/4 cup Gruyère per bowl if serving 6). Sprinkle a little Parmesan on top if using.

Melt cheese

Place the bowls on a baking sheet and broil 4–6 inches from the broiler element until the cheese is bubbling and deeply golden, about 3–5 minutes. Watch carefully to avoid burning. Alternatively, use a kitchen torch to evenly melt and brown the cheese.

Remove bowls from oven (use oven mitts). Let rest 1–2 minutes to avoid scalding. Garnish with freshly chopped parsley if desired and serve immediately.