French Onion Soup with Slow-Cooked, Golden Gruyère Toasts

French onion soup is a classic French dish of slowly softened, caramelised onions in savoury broth, served beneath toasted bread and bubbling cheese. The long, gentle cooking gives the onions deep sweetness, while dry wine and thyme balance the rich beef stock; regional and family versions vary in their choice of broth and cheese.

Ingredients

2½ lb (1.1 kg) yellow onions, peeled and thinly sliced
4 tbsp (56 g) unsalted butter
1 tbsp (15 ml) olive oil
1 tsp (5 g) fine salt, plus more to taste
2 tbsp (16 g) plain all-purpose flour
½ cup (120 ml) dry white wine
6 cups (1.4 L) low-sodium beef stock, preferably unsalted or lightly salted
1 bay leaf
4 fresh thyme sprigs, or ½ tsp (0.5 g) dried thyme
¼ tsp (0.5 g) freshly ground black pepper
1 tbsp (15 ml) sherry vinegar, or red wine vinegar
8 baguette slices, about ½ in (1.3 cm) thick
1½ cups (150 g) finely grated Gruyère cheese

Instructions

Prepare the onions and other ingredients. Peel the onions, halve them from root to tip, then slice them thinly from pole to pole. Keeping the slices a similar thickness helps them cook evenly. Measure out the stock and wine, grate the cheese, and have the thyme, bay leaf and vinegar ready. This preparation takes about 20 minutes.
Caramelise the onions. Set a large, heavy-based pot over medium heat and add the butter, olive oil and sliced onions. Sprinkle in the salt and stir to coat. Cook for 55 minutes, stirring every few minutes at first and more often as the onions soften and begin to colour. If they start catching on the base, lower the heat and stir in a tablespoon of water; scrape up the brown bits rather than letting them turn black. The onions are ready when reduced to a small, soft heap with an even deep golden-brown colour. Avoid rushing this stage with high heat: browned edges around still-firm onions taste harsher and less rounded.
Thicken the base and add the wine. Sprinkle the flour evenly over the onions and stir for 1 minute so it coats them and loses its raw, dusty taste. Pour in the wine gradually while stirring and scrape the base of the pot. Let it bubble for 2 minutes, until the sharp smell of alcohol has eased and the liquid has reduced slightly.
Simmer the soup. Add the beef stock, bay leaf, thyme and black pepper. Bring the soup just to a gentle boil, then lower the heat and simmer uncovered for 25 minutes. Keep it at a quiet simmer rather than a vigorous boil, which can make the broth cloudy and cook away too much liquid. Remove the bay leaf and thyme stems, stir in the vinegar, then taste and adjust the salt and pepper. Stock varies in saltiness, so wait until this point before adding much extra salt.
Toast the baguette. While the soup finishes, arrange the bread slices in one layer on a baking sheet. Toast under a hot grill or broiler, turning once, until crisp and lightly browned on both sides, about 8 minutes in total. Watch closely near the end; thin slices can go from golden to burnt quickly. Toasting dries the surface, helping it hold its shape longer once it meets the broth.
Top and broil the soup. Heat the grill or broiler to high. Ladle the hot soup into four heatproof oven-safe bowls, leaving a little room at the top. Set two toasted baguette slices on each portion and divide the Gruyère evenly over the bread, letting some cheese reach the edges. Place the bowls on a sturdy baking sheet and broil for about 3 minutes, watching continuously, until the cheese is melted, bubbling and browned in spots. Use oven mitts to remove the bowls and set them on heatproof plates. The bowls remain very hot; allow them to settle for a minute before serving.

Tips

Give the onions time. Their slow caramelisation is the heart of the soup. Stir often enough to prevent scorching, but not constantly; short periods of undisturbed contact with the pot help develop colour. If the onions are taking longer than expected, reduce the heat slightly and keep going rather than adding extra sugar to force the colour.
Choose stock thoughtfully. A good beef stock gives the most classic, full-bodied flavour. Use low-sodium stock so the soup does not become too salty as it reduces. For a vegetarian version, use a well-flavoured vegetable stock; the soup will be lighter and less savoury, so taste carefully and consider a little extra thyme or vinegar.
Use an alternative to wine if needed. Replace the white wine with the same amount of extra stock, then add a teaspoon of lemon juice or vinegar at the end if the soup needs brightness. The result will lack the wine’s aroma, but it will still be balanced. Sherry vinegar is traditional in some kitchens; red wine vinegar works well too.
Protect the cheese topping. Use oven-safe bowls and check their manufacturer’s guidance before putting them under a broiler. If you do not have suitable bowls, broil the cheese on the bread on a baking sheet and float the cheesy toasts on bowls of soup just before serving. This avoids heating non-broiler-safe dishes.

Serving Suggestions

Serve each bowl as soon as the cheese is bubbling, with the bread still crisp at the edges. Set the bowls on small plates to catch drips and warn everyone that the soup and dish are very hot. A simple green salad with a sharp vinaigrette is a useful counterpoint to the buttery onions and salty cheese; dress it lightly so it does not compete with the soup. For a larger meal, serve modest portions before roasted vegetables or a simple roast chicken, allowing the soup to remain the main flavour rather than adding another rich course.

Have extra toasted baguette on the table for anyone who wants to scoop up the broth after the first cheesy slice is gone. A little extra grated Gruyère can be offered separately, but avoid adding it to the soup after broiling, where it may clump instead of melting evenly. If serving several people, bring the bowls to the table on a tray and ladle carefully; individual bowls are easier to handle than one large, extremely hot casserole.

Variations

Make it vegetarian: Use vegetable stock instead of beef stock and check that the cheese is made without animal rennet if that matters to your guests. Gruyère is a classic topping, but a vegetarian-friendly aged Swiss-style cheese can stand in. Because vegetable stock can be less robust, use a well-seasoned one and adjust the vinegar and pepper after simmering.

Try another cheese: Comté, Emmental or a mature Swiss-style cheese melts well and brings a slightly different flavour. Gruyère is nutty and savoury; Emmental is usually milder and more stretchy. Avoid very soft fresh cheeses, which can turn oily or slide off the toast rather than forming a browned cap.

Use red wine: Replace the white wine with ½ cup (120 ml) dry red wine for a deeper, fruitier note and darker broth. Let it reduce for the same 2 minutes before adding the stock. The wine should support the onions rather than dominate them, so choose a dry wine and avoid sweet varieties.

Serve without a broiler-safe bowl: Make the soup in a regular pot, ladle it into serving bowls, and place a cheese-topped toasted baguette slice on each portion. The cheese will not have the same all-over browned crust, but the toast still softens at the bottom and keeps a crisp, cheesy top.

Storage and Reheating

For the best texture, store the soup and bread separately. Cool the soup and refrigerate it within 2 hours of cooking, in a covered container; it keeps for 3–4 days in the refrigerator. Refrigerate leftover toasted bread in a separate airtight container and use it within 1–2 days, though it may lose some crispness. Do not store soup in the bread-topped bowls, as the baguette will absorb the broth and become soft.

Reheat soup gently in a saucepan over medium-low heat, stirring occasionally until steaming hot throughout. Add a splash of stock or water if it has thickened in the refrigerator, then taste for seasoning. For food safety, USDA FSIS guidance is to reheat leftovers to 165°F (74°C); check the centre with a food thermometer. Toast fresh baguette, add cheese and broil just before serving. Freeze the soup without bread or cheese for up to 3 months in a freezer-safe container, leaving room for expansion. Thaw overnight in the refrigerator and reheat as above; the onions may soften further, but the flavour holds well.

Frequently Asked Questions

Why are my onions not turning brown?

They may need more time, or the pot may be too crowded for moisture to evaporate. Keep cooking over medium to medium-low heat, stirring and scraping the bottom, until the onions have reduced and become evenly golden-brown. If liquid gathers in the pot, continue cooking uncovered. A little water can loosen browned fond, but do not add so much that the onions stew for the rest of the process.

Can I make French onion soup ahead?

Yes. Cook the soup through the simmering step, cool it promptly and refrigerate it for up to 3–4 days. Reheat it until it reaches 165°F (74°C), then toast the bread and add the cheese immediately before serving. Keeping the topping separate prevents soggy bread and makes it easier to get a freshly browned finish.

Do I have to use Gruyère?

No. Gruyère is a classic choice because it melts smoothly and has a savoury, nutty flavour, but Comté, Emmental or another good melting Swiss-style cheese can work. Choose a cheese that browns rather than one that releases a lot of oil. The taste will change slightly, but the method stays the same.

Can I make the soup without alcohol?

Yes. Use ½ cup (120 ml) additional beef stock in place of the wine. After the soup simmers, add the measured vinegar and taste; a little extra can brighten the broth if it seems too sweet or flat. Do not add a large amount at once, since too much acidity can overpower the caramelised onions.

What if I do not have oven-safe soup bowls?

Do not put ordinary bowls under a broiler, where they could crack or break. Instead, broil the bread and cheese separately on a baking sheet, then place one cheesy toast on each bowl of hot soup. The soup will still taste rich and oniony, even without the traditional browned cheese cap.

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Safety notes: safe minimum internal temperatures from USDA FSIS; raw and undercooked fish guidance from the U.S. FDA. Food safety notes

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