French Onion Soup Gratinée is a classic bistro dish built on deeply caramelized onions simmered in rich beef stock, finished with garlic-rubbed croutons and a generous layer of melted Gruyère cheese. The long caramelization process and beef broth create its signature savory depth.
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Slice the onions thinly into half-moons.
In a large Dutch oven or heavy-bottomed pot, melt 1/2 cup of unsalted butter over medium to medium-low heat.
Add the sliced onions, season with kosher salt and pepper, and stir in the fresh thyme.
Sauté the onions for about 30 minutes until very tender. For the last 10 minutes, increase the heat to caramelize them.
Pour in the Cognac to deglaze the pot. Cook for a minute, but do not reduce it completely.
Add the beef stock, bring to a gentle bubble, then reduce the heat and simmer for 10 minutes.
While the soup simmers, preheat your oven to 375°F (190°C). Slice the baguette into 1/2-inch thick rounds.
Spread softened butter on one side of each baguette slice and season with kosher salt. Arrange on a baking sheet and bake until golden and toasted.
Once the toasts are out of the oven, rub the surface of each one with the large garlic clove.
Set your oven to broil. Ladle the hot soup into oven-safe bowls.
Sprinkle a small layer of Gruyère cheese over the soup, top with a few garlic toasts, then cover generously with the remaining Gruyère.
Place the bowls under the broiler for 3-5 minutes, or until the cheese is melted, bubbly, and golden brown.
Carefully remove from the oven, let cool slightly, garnish with a fresh thyme sprig if desired, and serve immediately.
Refrigerate soup (without croutons and cheese) for up to 3 days. Reheat gently on the stovetop and add fresh croutons and Gruyère topping before serving.
Heavy-bottomed pot or Dutch oven (at least 6-quart capacity for even caramelization) · Broiler-safe soup bowls or crocks · Broiler or oven set to high heat
Caramelizing 4-5 Spanish onions typically takes 45-60 minutes over medium-low heat, stirring occasionally, until deep golden brown. Low and slow is essential to avoid burning.
Yes. Prepare the soup up to 2 days ahead and refrigerate. Reheat gently on the stovetop, then add the croutons and Gruyère topping just before broiling.
Emmental, Comté, or Swiss cheese work well; aged cheddar is a firmer alternative but will change the flavor profile slightly.
Brandy or dry white wine can replace Cognac; use the same amount or reduce by half for white wine to avoid excessive acidity.
Day-old baguette slices are essential—they're firm enough to slice cleanly and toast without absorbing too much soup, creating a crisp base for the cheese.
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