French Onion Soup Recipe

Caramelize Onions

In a large heavy-bottomed pot or Dutch oven, heat butter and olive oil over medium heat until butter is melted. Add the thinly sliced onions, toss to coat, then sprinkle with coarse salt and the granulated sugar. Cook, stirring every few minutes, until onions soften and begin to brown, about 20–25 minutes. Reduce heat if they start to burn—you want deep golden brown, not black. Stir more frequently during final 10 minutes to ensure even caramelization.

Deglaze

When onions are richly browned, increase heat to medium-high and add the cognac (or dry sherry). Stand back slightly and stir to deglaze the pan, scraping up browned bits from the bottom. Let alcohol cook off for 1–2 minutes.

Build the Broth

Sprinkle the all-purpose flour over the onions and cook, stirring, for 1 minute to remove raw flour taste. Slowly add the beef broth and the optional beef stock/demi-glace while stirring to incorporate and avoid lumps. Add the bay leaf, thyme sprigs, Worcestershire sauce, and freshly ground black pepper.

Simmer

Bring soup to a gentle simmer, then reduce heat to low. Partially cover and simmer for 20–30 minutes to let flavors meld. Taste and adjust seasoning with salt and additional pepper if needed. Remove and discard thyme sprigs and bay leaf before serving.

Prepare Bread Slices

Preheat oven to 400°F (200°C) or set broiler to high. Arrange baguette slices on a baking sheet, brush lightly with oil or melted butter on both sides, and toast in the oven until golden and crisp, about 6–8 minutes, flipping once. If using broiler, watch carefully and toast 1–2 minutes per side.

Assemble for Serving (Oven Method)

Preheat oven to 400°F (200°C) and ladle soup into ovenproof bowls or crocks, filling each about three-quarters full. Place one or two toasted bread slices on top of each bowl, then pile a generous amount of grated Swiss cheese over the bread so it covers the surface. Sprinkle a little grated Parmesan on top if using. Place bowls on a baking sheet and bake until cheese is melted, bubbly and lightly browned, about 8–12 minutes.

Assemble for Serving (Broiler Method)

Alternatively, place toasted bread topped with cheese on the baking sheet and broil 2–4 minutes until cheese bubbles and browns; then transfer the cheesy toast to the soup bowls.

Carefully remove bowls from oven (they will be hot). Garnish with chopped parsley if desired. Serve immediately with spoon and place bowls on a protective surface.