French Onion Soup Recipe

Prepare onions

Peel the onions and cut off ends. Slice each onion in half through the root, then thinly slice into half-moons about 1/8 inch thick. Aim for uniform slices so they caramelize evenly.

Melt butter and oil

In a large, heavy-bottomed pot or Dutch oven over medium heat, add the unsalted butter and olive oil. Heat until butter is melted and foam has subsided, about 1 minute.

Sweat and caramelize onions

Add the sliced onions, sprinkle with sea salt, granulated sugar, and freshly ground black pepper. Stir to coat. Cook over medium to medium-low heat, stirring every 3–5 minutes to prevent sticking. After 15–20 minutes the onions will be soft and beginning to brown. Continue cooking another 20–30 minutes, reducing heat slightly if they begin to burn, until the onions are deep golden brown and jammy. This step can take 35–50 minutes total depending on heat and pot. Scrape browned bits from the bottom of the pot periodically.

Deglaze with wine

Increase heat to medium-high. Pour in the dry white wine or sherry and use a wooden spoon to scrape up all the fond (browned bits) from the bottom of the pot. Let the wine simmer until mostly evaporated, about 2–3 minutes.

Add stocks and aromatics

Add the beef stock and chicken stock to the pot. Stir in the bay leaves, fresh thyme sprigs, and Worcestershire sauce. Bring to a gentle simmer, then reduce heat to low. Simmer, uncovered, for 20–30 minutes to allow flavors to meld. Taste and adjust seasoning with salt and pepper as needed. Remove and discard thyme sprigs and bay leaves before serving.

Prepare bread

While the soup simmers, preheat the oven to 400°F (200°C). Arrange baguette slices on a baking sheet, brush lightly with olive oil, and toast until golden, about 6–8 minutes, flipping halfway. Alternatively, toast under a broiler watching closely to avoid burning.

Assemble and broil

Ladle the hot soup into oven-safe bowls or crocks, leaving about 1/2 inch headspace. Place one or two toasted baguette slices on top of each bowl. Mix the grated Gruyère with the Parmesan (if using) and sprinkle a generous layer over the bread so it covers the edges of the bowl. Place bowls on a baking sheet and broil in the oven on high for 2–4 minutes, watching carefully, until cheese is melted, bubbling, and lightly browned. If you prefer, use a kitchen torch to melt and brown the cheese instead.

Serve

Remove from oven (use oven mitts). Sprinkle with a pinch of fresh thyme leaves if desired. Let rest 1–2 minutes (very hot) then serve immediately.