French Onion Gratin Recipe

Prepare onions

Peel and cut the yellow onions in half from root to tip, then slice thinly (about 3–4 mm). Aim for even slices so they caramelize uniformly.

Heat a large, heavy-bottomed skillet or Dutch oven over medium heat. Add 1 tablespoon olive oil and 3 tablespoons unsalted butter. When the butter stops foaming, add the sliced onions and spread them in an even layer.

Sprinkle 1/2 teaspoon sugar, 1 teaspoon salt, and 1/2 teaspoon freshly ground black pepper over the onions. Cook, stirring every 3–5 minutes, adjusting the heat as needed so they brown slowly and do not burn. This step takes about 35–45 minutes total — the onions should become deeply golden to amber and jammy.

When the onions are deeply caramelized, add the minced garlic, 2 sprigs fresh thyme and 1 bay leaf. Cook for 1–2 minutes more until fragrant.

Increase heat to medium-high and add 120 ml dry white wine or dry sherry to deglaze the pan. Scrape any browned bits from the bottom with a wooden spoon. Let the wine reduce by about half (2–3 minutes).

Add 750 ml beef broth and 1 teaspoon Worcestershire sauce (if using). Stir, bring to a gentle simmer, then reduce heat to low and simmer uncovered for 10 minutes to meld flavors and reduce slightly. Taste and adjust seasoning with up to 1 teaspoon additional salt or 1 teaspoon sherry vinegar for brightness, if desired.

Remove and discard the thyme sprigs and bay leaf before assembling.

While the broth simmers, prepare the bread: preheat oven to 190°C (375°F). Arrange 6 slices of baguette on a baking sheet, brush lightly with 1 tablespoon melted butter, and toast until golden (6–8 minutes) or make croutons by cutting into cubes and toasting until crisp. Set aside.

Grate 2 cups Gruyère and 1/4 cup Parmesan (if using). Save a small handful (~2 tablespoons) of cheese for sprinkling at the very end.

Preheat broiler (or oven to 250°C/475°F with top rack near broiler). Place four 12–16 oz ovenproof bowls or one large gratin dish on a rimmed baking sheet to catch spills.

Divide the onion-broth mixture among the bowls, making sure each bowl gets some of the onions and enough hot broth to come close to the rim but not overflow (leave ~1 cm headspace). If broth is very thin, spoon more onions into each bowl so the topping sits on a mound of onions.

Top each bowl with 1–2 slices toasted baguette (or a few croutons) so they float on the surface, then generously heap grated Gruyère over the bread until fully covered. Sprinkle reserved cheese over the top for an even brown.

Place the baking sheet with bowls under the broiler. Broil 3–6 minutes, watching closely, until cheese is bubbly and golden brown, with some darker caramelized spots. Rotate pans if your broiler heats unevenly.

Remove from oven with oven mitts. Let cool 2–3 minutes — the gratin will be extremely hot. Serve immediately, garnished with a small thyme sprig if desired.