Real French onion soup lives or dies on the onions: cooked low and slow until they turn deep amber, they carry the whole pot without much else needed. A splash of wine, a good stock, and cheese-broiled bread floated on top turn one pot of onions into dinner.

One-Pot French Onion Soup with Broiled Gruyère Toast

Scale

Method

Melt the butter in a dutch oven over medium heat, then add the onions and salt, tossing to coat.
3 min
Cook the onions, stirring every few minutes and lowering the heat if they start to catch, until they collapse and turn a deep amber brown. This isn't a step to rush — add the sugar halfway through to help things along.
35 min
Stir in the garlic and cook until fragrant.
1 min
Sprinkle the flour over the onions and stir it in, cooking a minute or two to lose its raw taste.
2 min
Pour in the wine, scraping up any browned bits from the bottom of the pot, and simmer until it's mostly evaporated.
3 min
Add the stock, thyme and bay leaf. Bring to a simmer and cook uncovered until the broth tastes rounded rather than thin.
15 min
Discard the thyme stems and bay leaf, then season with the pepper.
1 min
Heat the broiler to high (about 500°F/260°C). Float the baguette slices on the surface of the soup, right in the pot, and scatter the cheese over them.
2 min
Broil until the cheese is melted and browned in spots, watching closely so it doesn't burn.
4 min

Substitutions

Dry white wine — Dry sherry, or skip it and deglaze with a splash of the stock plus a teaspoon of sherry vinegar.
Beef stock — Vegetable stock for a vegetarian soup — stir in a teaspoon of soy sauce or miso with the stock to keep the depth.
Gruyère — Swiss cheese, or a mix of mozzarella and grated Parmesan.
Baguette — Any crusty bread, sliced thick enough to hold its shape in the broth.

Storing leftovers

Keeps up to 4 days in the fridge once the bread and cheese are removed — the soup alone reheats well on the stove over medium-low heat. Broiled bread turns soggy if stored in the soup, so keep leftover toast separate and give it a couple of minutes back under the broiler when reheating.

Questions

Why do the onions take 35 minutes?

That's how long it takes to cook off their water and let the natural sugars caramelize. Rushing it with high heat browns the outside before the inside softens, and you get bitter spots instead of even amber colour.

Can I make this vegetarian?

Yes — use vegetable stock and stir in a teaspoon of soy sauce or miso paste with it to replace some of the savoury depth beef stock would normally give.

My dutch oven isn't broiler-safe — what do I do?

Toast the bread separately under a broiler or in a toaster oven, melt the cheese onto it there, then float the toast on top of the soup just before serving.

Can I make the onions ahead?

Yes — caramelized onions keep in the fridge up to 3 days. Reheat them in the pot before adding the flour and continuing with the recipe.