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Ingredients
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French Onion Soup Base
Crostini & Cheese Topping
Finishing (optional)
Method
French Onion Soup Base
Prepare onions
Peel the onions and cut off the ends. Slice them in half from root to tip, then thinly slice across into half-moons about 1/8-inch thick. Aim for even slices so they caramelize consistently.
Caramelize onions
Heat a large, heavy-bottomed pot or Dutch oven over medium heat. Add the butter and oil; when the butter foams and just begins to brown, add the sliced onions. Stir to coat them in fat. Sprinkle the salt and sugar over the onions (the sugar helps with browning). Reduce heat to medium-low. Cook the onions, stirring every 4–5 minutes and scraping the fond from the bottom, until they are deep golden brown and richly caramelized—this takes about 40–50 minutes. If the pan dries out or the onions begin to stick/burn, add a tablespoon or two of water and scrape the brown bits up.
Add garlic and deglaze
When the onions are deeply caramelized, add the minced garlic and cook 30–45 seconds until fragrant. Increase heat to medium-high and pour in the white wine or sherry to deglaze, scraping up any browned bits. Let the alcohol simmer off and reduce by about half, 2–3 minutes.
Simmer with stock
Add the beef stock, bay leaves, and thyme sprigs. Bring to a gentle simmer, then reduce heat to low and simmer uncovered for 20 minutes to meld flavors. Stir in Worcestershire sauce and black pepper. Taste and, if desired, add the optional splash of sherry or 1 tsp sherry vinegar to brighten. Remove and discard the bay leaves and thyme stems before serving.
Crostini & Cheese Topping
Toast baguette slices
Preheat the oven to 375°F (190°C). Arrange baguette slices in a single layer on a baking sheet. Brush the tops lightly with olive oil. Toast in the oven for 8–10 minutes, flipping once, until golden and crisp. Alternatively, broil briefly—watch closely to prevent burning.
Rub with garlic
As soon as the crostini come out of the oven, rub the top of each slice with the halved garlic clove to impart a subtle garlic aroma.
Assemble cheese-top crostini
Place or mound a spoonful of caramelized onion on each crostini, top with grated or sliced Swiss cheese. If finishing under a broiler or salamander: arrange topped crostini on a baking sheet and broil 2–3 minutes until the cheese is melted and bubbly with light browning. Alternatively, place soups bowls under broiler in the next step (see serving).
Finishing & Serving
Season soup to taste
Taste the soup and adjust seasoning with salt and additional pepper if needed. If using, add the optional 1 tsp vinegar for brightness and stir.
Serve traditional (in bowls) or deconstructed
Traditional method: Preheat broiler. Ladle hot soup into oven-safe bowls, float a crostini on each bowl, top with generous slices or shreds of Swiss cheese to cover the crostini, and place bowls on a baking sheet. Broil 2–4 minutes until cheese is bubbly and golden. Remove carefully and garnish with snipped chives. Deconstructed method: Serve soup in bowls with crostini topped with cheese and chives on the side so diners place them themselves.
Safety & final notes
Use oven mitts and a sturdy tray when moving hot bowls. Leftover soup keeps 3–4 days refrigerated and reheats gently on the stovetop. Remove thyme stems before storing for best texture.