French Onion Soup Recipe

Prepare onions

Peel and thinly slice the yellow onions into half-moons about 1/8-inch thick so they cook evenly.

Caramelize onions

In a large heavy-bottomed pot or Dutch oven, melt the butter with the 1 tbsp olive oil over medium heat. Add the sliced onions, sprinkle the 1 tsp sugar and 1 tsp salt over them, and stir to coat. Cook, stirring every 3–5 minutes, adjusting heat to maintain a steady gentle bubble but not burning, for 35–45 minutes until onions are deeply golden-brown and sweet. If onions begin to stick too aggressively, lower the heat and add a splash (1–2 tbsp) of water to deglaze the pan.

Add flour and deglaze

Push the onions into the center, sprinkle 1 tbsp flour over them, stir and cook 1–2 minutes to remove the raw flour taste. Pour in the 1/2 cup dry white wine (if using) and scrape the brown bits from the bottom with a wooden spoon. Let the wine reduce for 1–2 minutes.

Simmer soup

Add the 6 cups beef stock and 1 cup chicken stock (or an extra cup beef stock), stir in the bay leaf, thyme sprigs, 1 tsp Worcestershire sauce, and 1/2 tsp freshly ground black pepper. Bring to a gentle simmer, then reduce heat and simmer uncovered for 20–25 minutes to meld flavors. Taste and adjust seasoning with additional salt and pepper as needed, removing thyme sprigs and bay leaf before serving.

Prepare baguette slices

Preheat oven to 400°F (200°C). Arrange baguette slices on a baking sheet. Brush each slice lightly with olive oil on both sides. Toast in the oven for 6–8 minutes, flipping once, until crisp and lightly golden.

Rub with garlic

As soon as the toasts come out of the oven, rub the cut side of the halved garlic clove over the top of each warm slice to impart a mild garlic flavor.

Top with cheese

Heap shredded Swiss (Gruyère or Emmental) onto each crostini; sprinkle a little grated Parmesan on top if using. Return crostini to the oven (or under a broiler) and melt until cheese is bubbly and lightly browned, about 2–4 minutes under broiler — watch carefully to avoid burning.

Ladle the hot soup into oven-safe bowls or crocks, filling them about three-quarters full.

Place one or two cheesy crostini on top of each bowl of soup. If you prefer the cheese fully melted over the soup, top each bowl with a slice of baguette covered in cheese and place under a broiler until bubbly and golden (about 2–4 minutes).

Garnish with chopped parsley and additional cracked black pepper to taste. Serve immediately.