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Ingredients
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Soup Base
Gruyère Toast (Croûtons and Topping)
Finishing & Seasoning
Method
Soup Base
Prepare and caramelize onions
In a large heavy-bottomed pot or Dutch oven over medium heat, melt 3 tablespoons butter with 1 tablespoon olive oil. Add the thinly sliced onions, 1 teaspoon salt, 1 teaspoon freshly ground black pepper and 1 teaspoon granulated sugar. Toss to coat and spread onions evenly in the pot. Cook, stirring every 4–5 minutes, adjusting heat as needed so onions brown slowly without burning. This process should take 35–45 minutes until onions are deeply golden-brown and caramelized.
Deglaze
Increase heat to medium-high, add 1 cup dry white wine (or vermouth) to deglaze the pot, scraping up browned bits from the bottom with a wooden spoon. Let the wine reduce by about half, about 2–3 minutes.
Build the soup
Stir in 1 tablespoon tomato paste and cook for 1 minute. Add 6 cups beef stock and 1 cup chicken stock, then add the thyme sprigs and 2 bay leaves. Bring the soup to a gentle simmer, reduce heat to low, and simmer uncovered for 20–25 minutes to let flavors meld.
Finish and season
Remove and discard thyme sprigs and bay leaves. Stir in 2 teaspoons sherry or Cognac if using. Taste and adjust seasoning with additional salt and freshly ground black pepper as needed.
Gruyère Toast (Croûtons and Topping)
Prepare baguette slices
Preheat the oven to 375°F (190°C). Arrange the baguette slices on a baking sheet. Brush each slice lightly on both sides with softened butter (use about 2 tablespoons total). Toast in the oven until just crisp and golden, about 8–10 minutes, flipping once halfway through. Remove from oven and rub the top of each warm toast with the halved garlic clove for subtle garlic flavor.
Assemble cheese toasts
Pile about 1/5 cup grated Gruyère (approx. 2 cups total divided across 10 slices) on each toast so cheese mounds slightly. Sprinkle each with a little grated Parmesan if using.
Melt cheese
Switch the oven to broil (high). Place the cheese-topped toasts on the baking sheet under the broiler until the Gruyère is melted, bubbling and just beginning to brown, 1–3 minutes — watch closely to prevent burning. Remove and set aside.
Assembly & Serving
Ladle the hot soup into oven-safe bowls or crocks, filling them about three-quarters full to allow room for the cheese toast and bubbling. Float one or two Gruyère toasts on top of each bowl (or place the toast on top of the bowl and transfer to a baking sheet), then place bowls on a baking sheet and put under the broiler until the cheese is bubbly and golden and the soup at the surface is gently bubbling, about 2–4 minutes. Alternatively, place the toast on top of the bowls and broil briefly just to brown the cheese. Remove carefully from the oven.
Serve immediately with extra freshly ground black pepper and additional grated Gruyère if desired. Enjoy hot.