French Onion Soup au Gratin

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Published July 31, 2024  ·  ⭐ 4.0 (10 reviews)

Prep	Cook	Total	Yield
20 min	1 hr 30 min	2 hr	4 servings

Difficulty: Medium

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Ingredients

Soup

• 1.5 kg yellow onions, thinly sliced
• 60 g unsalted butter
• 2 tbsp olive oil
• 1 tsp fine sea salt
• 1 tsp caster sugar
• 4 cloves garlic, minced
• 200 ml dry white wine or dry sherry
• 1.5 litres good beef stock
• 2 sprigs fresh thyme
• 2 bay leaves
• 1 tbsp plain flour
• Salt and black pepper

Gratin Topping

• 8 thick slices baguette
• 1 clove garlic
• Olive oil
• 180 g Gruyère, coarsely grated

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Steps Overview

1. Caramelise onions very slowly (60–75 minutes)
2. Add wine, stock, and herbs; simmer
3. Toast the baguette croutons
4. Ladle into oven-safe bowls
5. Top with crouton and cheese, grill until bubbling and brown

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Step-by-Step

1. Caramelise the onions

In a large heavy-based pot, melt butter with olive oil over medium heat. Add onions, salt, and sugar. Cook, stirring every 5–10 minutes, for 60–75 minutes. Lower heat if they start to catch. The onions are ready when they are a deep, uniform caramel brown, extremely soft, and reduced to about a quarter of their original volume. There is no shortcut for this step.

2. Build the soup

Add garlic and cook 2 minutes. Sprinkle in flour and stir for 1 minute. Pour in the wine and scrape up any sticky bits. Add stock, thyme, and bay leaves. Simmer, partially covered, for 30 minutes. Remove herbs, season generously.

3. Make the croutons

Rub baguette slices with the cut garlic clove, brush with olive oil, and grill or toast until golden on both sides.

4. Assemble and grill

Preheat grill to high. Ladle soup into oven-safe bowls. Place on a baking tray. Float one or two croutons on top of each. Pile Gruyère generously over the croutons and the edges of the soup. Grill for 3–5 minutes until the cheese is bubbling, spotted brown, and starting to caramelise at the edges.

5. Serve

Carry bowls carefully to the table — they are extremely hot. Serve immediately.

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Notes

• Patience with the onions is the whole recipe. Rushing them produces sweet but not complex soup.
• Beef stock is traditional and essential for depth. Use the best quality you can find.
• Gruyère is classic, but Comté or Emmental also work beautifully.
• The soup base freezes perfectly for up to 3 months without the crouton topping.