Classic French Onion Soup Gratinée with Gruyère

Why You’ll Love This Recipe

• Deep, restaurant-level flavor from slowly caramelized onions and a proper deglaze.
• Broiled, bubbly Gruyère cap over crisp baguette makes every spoonful cheesy and satisfying.
• Approachable, step-by-step method with exact times and temperatures for foolproof results.
• Great make-ahead base; broil to order for guests or cozy weeknights.

Grocery List

• Produce: Yellow onions (3 lb/1.4 kg), garlic (2 cloves), fresh thyme, bay leaf
• Dairy: Unsalted butter, Gruyère cheese (8 oz/225 g)
• Pantry: Olive oil, all-purpose flour, dry white wine or dry sherry, low-sodium beef stock, Worcestershire sauce, sherry or red wine vinegar, fine sea salt, black pepper, sugar (optional), baking soda (optional), baguette

Full Ingredients

For the Soup

• 3 lb (1.4 kg) yellow onions, halved pole-to-pole and thinly sliced
• 3 tbsp unsalted butter
• 1 tbsp olive oil
• 1 1/4 tsp fine sea salt, divided
• 1 tsp granulated sugar (optional, encourages browning)
• Pinch baking soda (optional, speeds caramelization—use sparingly)
• 2 garlic cloves, minced
• 1 tbsp all-purpose flour
• 1/2 cup (120 ml) dry white wine or dry sherry
• 6 cups (1.4 L) low-sodium beef stock
• 2 fresh thyme sprigs
• 1 bay leaf
• 1 tsp Worcestershire sauce (optional, for umami)
• 1 tsp sherry vinegar or red wine vinegar
• Freshly ground black pepper, to taste

For the Gratinée

• 1 narrow baguette, cut into 12 slices (1/2 in / 1.25 cm thick)
• 1 tbsp olive oil (for brushing, optional)
• 8 oz (225 g) Gruyère, coarsely grated

Step-by-Step Instructions

Step 1: Slice the onions and set up

Halve onions from root to stem, peel, then slice thinly pole-to-pole so they soften evenly without turning mushy. Place a large, wide Dutch oven or heavy pot (at least 5 quarts) over medium heat. Add the butter and olive oil.

Step 2: Sweat, then deeply caramelize

Add onions, 1 teaspoon of the salt, and the optional sugar and baking soda. Stir to coat, then cook over medium heat for 5–7 minutes until they release liquid. Reduce to medium-low and cook, stirring every 5 minutes and scraping the fond, until the onions are a deep mahogany brown, 45–60 minutes. If browning too quickly, lower heat and add 1–2 tablespoons water to loosen the fond.

Step 3: Add aromatics and flour

Stir in minced garlic and cook 30 seconds until fragrant. Sprinkle the flour over the onions and cook, stirring, for 1 minute to remove the raw taste. This helps the broth thicken slightly and cling to the onions.

Step 4: Deglaze with wine

Pour in the wine (or sherry) and scrape the bottom to dissolve the fond. Simmer 1–2 minutes until the harsh alcohol smell dissipates and the liquid reduces by about half.

Step 5: Build and simmer the soup

Add the beef stock, thyme sprigs, bay leaf, and Worcestershire (if using). Bring to a simmer over medium heat, then reduce to medium-low and simmer uncovered for 25–30 minutes. The broth should become glossy and slightly syrupy around the onions.

Step 6: Toast the baguette croûtes

While the soup simmers, heat the oven to 375°F (190°C). Arrange baguette slices on a baking sheet. Brush lightly with olive oil if desired. Bake 10–12 minutes, flipping once, until fully dry and crisp. This ensures they stay crunchy under the cheese.

Step 7: Season and prepare to broil

Remove thyme and bay. Stir in the vinegar, several grinds of black pepper, and the remaining 1/4 teaspoon salt (or adjust to taste). Set the oven rack 6 inches (15 cm) from the broiler element and switch the broiler to high.

Step 8: Assemble and gratinée

Place broiler-safe soup crocks on a rimmed baking sheet. Ladle hot soup into each bowl. Float 2 baguette slices on each portion and top generously with Gruyère (about 1 1/4–1 1/2 oz per bowl). Broil on high for 3–5 minutes, watching closely, until the cheese is melted, bubbling, and browned in spots. Let rest 2–3 minutes before serving; bowls will be very hot.

Pro Tips

• A wide pot caramelizes faster and more evenly than a narrow one by maximizing surface area.
• If fond gets too dark, splash in a tablespoon of water and scrape; controlled deglazing builds complex flavor.
• Use low-sodium stock so you can season precisely at the end.
• For extra-gooey melt, toss grated Gruyère with 1 tsp cornstarch before topping.
• Place crocks on a baking sheet for safe transfer in and out of the oven.

Variations

• Classic Parisian: Add 1–2 tbsp Cognac or brandy to the pot after deglazing with wine; simmer 1 minute.
• Vegetarian: Swap beef stock for rich vegetable stock; add 1 tsp white or red miso for umami depth.
• Onion Blend: Use a mix of yellow, sweet, and a little red onion for layered sweetness and color.

Storage & Make-Ahead

The soup base (without bread and cheese) keeps 4 days in the refrigerator or up to 3 months in the freezer. Reheat to a simmer, adjust seasoning, then assemble with fresh toasts and Gruyère and broil to serve. Store toasted baguette in an airtight container at room temperature for up to 3 days; grate cheese up to 2 days ahead and refrigerate.

Nutrition (per serving)

Approximate: 485 calories; 25 g fat; 38 g carbohydrates; 22 g protein; 3 g fiber; 980 mg sodium. Values will vary with stock and cheese choices.