French Onion Soup Recipe

Prepare onions

Peel and cut the yellow onions in half from root to tip. Lay each half flat and slice thinly (about 1/8-inch). You should have roughly 8–10 cups of sliced onions from 2.5 lb.

Cook onions low and slow

In a large, heavy-bottomed pot or Dutch oven, melt the butter with the olive oil over medium heat. Add all sliced onions, sprinkle the salt and sugar over them, and toss to coat. Spread the onions evenly across the bottom of the pot and cook, stirring every 5–8 minutes, until they become deeply golden brown and jammy — about 40–50 minutes. Reduce heat as needed to prevent burning; the goal is even caramelization.

Deglaze

When the onions are a deep golden-brown, increase heat to medium-high and add the dry white wine (or sherry). Scrape the browned bits from the bottom of the pot with a wooden spoon. Let the alcohol cook off and the liquid reduce by half, about 2–3 minutes.

Add stock and aromatics

Add the beef stock and chicken stock to the pot along with the thyme sprigs and bay leaves. Bring the mixture to a simmer, then reduce heat to low and simmer gently, uncovered, for 20–30 minutes to let flavors meld. Taste and adjust seasoning with additional salt and pepper if needed. Remove and discard thyme sprigs and bay leaves before serving.

Prepare croutons

While the soup simmers, preheat the oven to 400°F (200°C). Lightly brush both sides of the baguette slices with olive oil or melted butter and place on a baking sheet. Toast in the oven until lightly golden, about 6–8 minutes, flipping halfway. For an extra-crispy base, toast 1–2 minutes longer.

Assemble bowls

Ladle the hot soup into ovenproof bowls or small crocks, filling them about 3/4 full. Place 1–2 toasted baguette slices on top of each bowl of soup (depending on bowl size). Generously mound grated Gruyère over the bread so it covers the bread and a little of the soup surface. Sprinkle the optional Parmesan on top if using.

Broil until bubbly

Place the bowls on a baking sheet and set under a preheated broiler on high (or 500°F/260°C) about 6 inches from the heat source. Broil for 2–4 minutes until the cheese is melted, bubbling, and slightly browned. Watch carefully to avoid burning.

Finish and serve

Remove bowls from the oven using oven mitts. Sprinkle with the optional fresh thyme leaves. Let cool 2–3 minutes, then serve hot. If preferred, ladle soup into bowls and top with the toasted bread and cheese without broiling (for a less browned but still melted result).