French onion soup au gratin (Soupe à l’oignon)

Ingredients

• 1 kg of yellow or white onions, about 6 large ones, halved and thinly sliced

• 40 g unsalted butter

• 1 tablespoon of olive oil, 15 ml

• 1 level teaspoon of salt, 5 g, divided, plus as needed to adjust

• 1 tablespoon of wheat flour, 10 g

• 120 ml dry white wine

• 1.2 liters of hot meat broth, preferably lightly salted

• 3 sprigs of fresh thyme

• 1 bay leaf

• Freshly ground black pepper to taste

• 8 slices of baguette, about 1.5 cm thick

• 1 clove of garlic, cut in half

• 200 g coarsely grated Gruyère cheese

Preparation method

1. Slice and start sweating the onions

Use a wide, thick-bottomed pan. Melt the butter with the olive oil over medium heat, add the onions and half the salt and stir to coat. Cook for about 10 minutes, stirring occasionally, until the onions soften and release liquid.

2. Caramelize without sugar

Reduce heat to medium-low and cook for another 35 to 45 minutes. At first, stir every 5 minutes; When the onions start to brown, stir more frequently and scrape the bottom. If dark spots start to burn before the rest turns color, add 1 tablespoon of water and scrape the bottom. The goal is to get onions that are soft, sticky, and golden brown, not black or crunchy.

3. Cook the flour and deglaze

Sprinkle the flour over the onions and stir for 1 to 2 minutes, until the raw appearance disappears. Add the white wine and vigorously scrape the bottom of the pan to dissolve the caramelized layer. Boil for 2 to 3 minutes, until the wine has reduced by approximately half.

4. Form the soup broth

Add the hot beef broth, thyme, bay leaf and black pepper. When it boils, lower the heat and cook uncovered for about 20 minutes, at a gentle boil. Remove thyme and bay leaves. Taste before adding the rest of the salt, because the concentration of the broth and the cheese already bring saltiness.

5. Dry and brown the baguette

While the soup is cooking, heat the oven to 200°C. Place the baguette slices on a baking tray and bake for 8 to 10 minutes, turning halfway through, until dry and golden on the edges. Lightly rub the cut side of the garlic over the still hot slices. Toasting is important for the bread to support the cheese without immediately falling apart in the broth.

6. Assemble the bowls

Increase the oven to 240°C or heat the grill to high power. Place 4 heatproof bowls on a baking sheet. Distribute the soup very hot, leaving about 2 cm free to the edge. Place 2 slices of baguette in each bowl and cover with Gruyère, spreading the cheese close to the edges.

7. Brown until really golden brown

Place the bowls in the very hot oven for 5 to 10 minutes, or under the grill for as long as necessary, until the cheese melts completely, bubbles and takes on golden brown spots. The broth should bubble to the sides. Carefully remove the baking tray and let it rest for 5 minutes before serving; the bowls remain very hot.

Right spot / how to know it’s ready

The onions ready to receive the wine must be reduced to a fraction of their initial volume, very soft, shiny and a uniform golden brown tone, with a caramelized base that releases when scraping the pan. After the broth, the soup should appear bright and have a concentrated onion flavor, without a raw taste of flour or wine. In the end, the cheese needs to be completely melted and bubbly, with nicely browned areas, while the baguette still maintains structure under the topping.

Tips for the recipe to work out

Use a wide pan

The larger the contact area, the faster the water released by the onions evaporates. In a narrow pan, they tend to cook in their own steam for a long time before starting to brown.

Control the heat instead of adding sugar

The necessary sweetness appears with the natural caramelization of the onion. If the bottom darkens too quickly, reduce the heat and use small amounts of water to loosen the caramelized residue without letting it burn.

Start with a tasty, slightly salty broth

The soup reduces during cooking and adds cheese at the end. A broth that is already very salty may become unbalanced after concentration. If the broth is light, let the soup reduce for a few more minutes before adjusting the salt.

Toast the bread until it loses moisture

Baguette that has just been heated absorbs the juice quickly and can sink to the bottom before browning. The slice should be dry in the center and golden around the edges before adding to the bowl.

Gratine only when serving

You can prepare the soup base in advance, but assemble the bread and cheese only with the soup again very hot and immediately before putting it in the oven. This preserves the contrast between broth, bread and topping.

Substitutions and variations

Gruyère can be replaced with Emmental or Comté. Emmental tends to be milder; Comté tends to have a more pronounced flavor. A national Gruyère cheese also works if it melts well and browns under the grill.

For a version without wine, deglaze with 120 ml of the broth itself. At the end of cooking, a few drops of white wine vinegar can recover some of the acidity, but the flavor becomes less vinous and complex.

Chicken broth can replace beef broth and produces a lighter soup. In this case, it is worth reducing it a little more to obtain intensity before assembling.

How to store

Preferably only keep the soup base, without bread or cheese. Transfer to shallow containers and refrigerate within 2 hours of preparation. Keep refrigerated at 4°C or below and consume within 3 to 4 days.

To freeze, wait for the base to cool under refrigeration, portion into suitable containers and freeze without the toast or cheese. For best quality, use within about 2 months. When reheating, bring the soup to a full boil; If measuring with a thermometer, the leftovers must reach at least 74°C. Only then assemble it with baguette and cheese and make the fresh gratin.

How to serve / side dishes

Serve immediately in refractory bowls, supported on heat-resistant plates. Because the soup is intense and the gratin is rich, a leafy salad with an acidic dressing works well as a simple side dish. Tell the table that the dishes come out of the oven very hot.

Common mistakes

Why are my onions burnt and still firm?

The heat was too high for the caramelization time. Lower the heat once the onions have softened, stir with increasing frequency and loosen the bottom with small amounts of water when necessary.

Why was the soup watery?

Normally there was a lack of caramelization or reduction. The onions need to lose a lot of water and gain color before the wine; After adding the broth, maintain a gentle simmer uncovered until the flavor is concentrated.

Why did the bread fall apart?

The baguette was probably fresh or slightly toasted. Use slices with good structure and bake until dry in the center before assembling. Don't leave the assembled soup waiting to go in the oven.

Why did the cheese melt but not brown?

The upper heat may be weak or too far away. Use a very hot grill or oven, place the bowls on a safe baking tray and monitor closely until bubbles and golden brown spots appear.