French Onion Soup Recipe

Prepare onions

Warm a large heavy-bottomed pot or Dutch oven over medium heat. Add butter and 1 tablespoon olive oil. When butter melts and begins to foam, add the thinly sliced onions in an even layer. Sprinkle with 1 tsp fine sea salt and 1 tsp sugar (this helps with caramelization). Cook, stirring every few minutes, for 20–30 minutes until onions are deep golden brown and very soft. If onions start to darken too quickly, lower the heat. Adjust heat to maintain a steady, gentle caramelization.

Deglaze and simmer

Raise heat to medium-high and add 1 tablespoon flour to the onions; cook 1–2 minutes, stirring, to remove raw flour taste and help thicken. Carefully pour in the dry sherry to deglaze, scraping up browned bits from the pot with a wooden spoon. Let the sherry reduce by half, about 2–3 minutes.

Build the broth

Add the beef stock, beef bouillon paste (if using), Worcestershire sauce, thyme sprigs, and bay leaves. Bring to a gentle simmer, then reduce heat to low and simmer uncovered for 20 minutes to meld flavors. Taste and season with coarse sea salt and additional black pepper as needed. Remove and discard thyme sprigs and bay leaves before serving.

While soup simmers, preheat oven to 400°F (200°C). Arrange baguette slices on a baking sheet and brush lightly with remaining 2 tsp olive oil on both sides. Toast in oven until golden and crisp, about 6–8 minutes, flipping once. Optional: rub the warm toasted slices with the cut side of the garlic clove for a subtle garlic note.

Mix Gouda, provolone, pizza mozzarella, and grated Parmesan in a bowl until evenly combined. Portion the shredded cheese so each serving of soup will get a generous layer (about 3/4 to 1 cup of mixed cheese per bowl).

Preheat broiler to high (or oven to 450°F/230°C if broiler not available). Ladle hot soup into oven-safe bowls or crocks, leaving about 1/2-inch headspace. Place 1–2 crostini slices on top of each bowl (or set the crostini on the side if preferred). Mound the mixed cheeses over the crostini (or directly on the soup) to form an even layer covering the surface.

Place bowls on a baking sheet and broil on the top rack until cheese is fully melted, bubbling, and golden brown in spots, about 3–6 minutes depending on broiler strength. Watch carefully to avoid burning. Alternatively, bake at 450°F (230°C) for 8–12 minutes until cheese is browned.

Carefully remove bowls from oven (they will be very hot). Sprinkle each with a pinch of chopped parsley, a light dusting of coarse sea salt, and a crack of fresh black pepper. Let cool slightly (2–3 minutes) before serving to avoid scalding.