French Onion Soup Gratinée with Gruyère

Why You’ll Love This Recipe

• Slow-caramelized onions build deep, authentic bistro flavor—no shortcuts.
• A bubbling Gruyère cap turns the soup into a cozy gratin with crisp, lacy edges.
• Clear, reliable timing and temperatures make restaurant-quality results at home.
• Great for make-ahead: the soup base keeps beautifully, then you gratinée to order.

Grocery List

• Produce: 2 1/2 lb yellow onions, 1 garlic clove, fresh thyme, 1 bay leaf
• Dairy: 8 oz Gruyère, unsalted butter
• Pantry: Olive oil, kosher salt, black pepper, sugar (optional), dry white wine, dry sherry or brandy (optional), low-sodium beef stock, Worcestershire (optional), sherry or red wine vinegar, baguette

Full Ingredients

For the Caramelized Onions

• 2 1/2 lb (1.1 kg) yellow onions, halved pole-to-pole and thinly sliced 1/8 inch (3 mm)
• 3 tbsp unsalted butter
• 1 tbsp olive oil
• 1 1/2 tsp kosher salt (use 1–1 1/4 tsp if using fine sea salt)
• 1/2 tsp sugar (optional, helps start caramelization)

For the Soup

• 1/2 cup (120 ml) dry white wine
• 2 tbsp dry sherry or brandy (optional but classic)
• 6 cups (1.4 L) low-sodium beef stock (or 4 cups beef + 2 cups chicken for balance)
• 4 fresh thyme sprigs (or 1 tsp dried thyme)
• 1 bay leaf
• 1/2 tsp freshly ground black pepper
• 1 tsp Worcestershire sauce (optional)
• 1 tsp sherry vinegar or red wine vinegar, to finish

For the Gratinée

• 12 baguette slices, 1/2 inch (1.25 cm) thick
• 1 garlic clove, halved lengthwise
• 8 oz (225 g) Gruyère, coarsely grated (about 2 packed cups)

Step-by-Step Instructions

Step 1: Slice the onions

Halve onions through the root, peel, then slice thinly pole-to-pole about 1/8 inch (3 mm) thick. Uniform slices ensure even caramelization and silky texture in the finished soup.

Step 2: Sweat the onions

In a wide, heavy pot or Dutch oven (at least 5 quarts), melt the butter with the olive oil over medium-low heat. Add onions, kosher salt, and optional sugar. Stir to coat, then cover for 5 minutes to start softening. Uncover and continue cooking, stirring every few minutes.

Step 3: Caramelize to deep mahogany

Maintain medium-low heat and cook 45–55 minutes, stirring often and scraping the bottom to prevent scorching. Adjust heat up or down so the fond browns but does not burn. Toward the end, the onions should be jammy, reduced in volume, and a deep chestnut color with a glossy sheen.

Step 4: Deglaze with wine and sherry

Raise heat to medium. Add the white wine (and optional sherry or brandy). It will hiss. Scrape up all browned bits until the bottom is clean. Reduce by about half, 2–3 minutes, to concentrate flavor.

Step 5: Build and simmer the soup

Stir in the stock, thyme sprigs, bay leaf, black pepper, and optional Worcestershire. Bring to a gentle simmer and cook uncovered for 25–30 minutes. Remove thyme and bay. Taste and adjust salt; finish with the vinegar to brighten the caramel notes.

Step 6: Toast the baguette

While the soup simmers, heat the oven to 375°F (190°C). Arrange baguette slices on a sheet pan and bake until crisp and lightly golden, 8–10 minutes. Rub one side of each toast with the cut side of the garlic clove. Increase oven to High broil for the final step.

Step 7: Assemble and broil the gratinée

Set oven-safe bowls on a rimmed sheet pan. Ladle in hot soup. Float two garlic-rubbed toasts in each bowl. Generously mound with Gruyère, letting the cheese touch the bowl’s rim to form a seal. Broil on the top rack until the cheese is bubbling, browned in spots, and lacy at the edges, 3–5 minutes. Let rest 5 minutes before serving—cheese will be molten.

Pro Tips

• Use a wide pot so moisture evaporates and onions truly caramelize, not steam.
• Control heat: if onions start to scorch, add a splash of water and lower the heat.
• For deeper savoriness, use a 2:1 mix of beef to chicken stock and add the optional Worcestershire.
• Grate cheese on the coarse side; it melts into gooey strands while still browning beautifully.
• Place bowls on a preheated sheet pan for even broiling and safer handling.

Variations

• Vegetarian: Use rich mushroom stock; add 1 tsp soy sauce for umami and 1 tsp balsamic to finish.
• Savoyard Twist: Swap some Gruyère for Comté and add a pinch of nutmeg to the soup.
• Cider Onion Soup: Deglaze with dry hard cider instead of wine and finish with apple cider vinegar.

Storage & Make-Ahead

The soup base (without bread and cheese) keeps 4 days refrigerated or up to 3 months frozen. Cool completely before storing. Reheat gently on the stovetop, then assemble with fresh toasts and cheese and broil before serving. Toasts can be made 1 day ahead and kept airtight. Gratinée is best eaten fresh.

Nutrition (per serving)

Approximate: 420 calories; 18 g protein; 18 g fat; 39 g carbohydrates; 3 g fiber; 1080 mg sodium (varies with stock and salt).