French Onion Soup: Deep, Sweet and Bubbling with Cheese

French onion soup is a bowl of patience rewarded: a pile of plain onions cooked slowly until they turn deep brown and sweet, simmered in savory broth and topped with a slice of toasted bread under bubbling, browned cheese. It started as a humble Paris bistro dish and is now one of the most searched comfort soups every winter, because it tastes far more impressive than its short ingredient list suggests.

The only real secret is time. Onions need about 45 minutes over medium heat to caramelize properly, but they need very little attention. Everything else is easy, and the soup keeps well in the fridge for days, so it is perfect to make ahead for a dinner party or a cold week.

Why this recipe works

• Plenty of onions. Six large yellow onions look like far too many, but they cook down to about a quarter of their volume. Yellow onions have the right balance of sugar and sharpness for deep flavor.
• Butter plus a little oil. The oil raises the smoke point so the butter does not burn while the onions brown.
• A splash of wine and a splash of vinegar. Wine lifts the browned bits off the pot, and a little sherry vinegar at the end brightens the sweetness.
• Gruyère on a toasted baguette. It melts smoothly, browns well and has a nutty flavor that suits the sweet onions.

Tips for the best onion soup

• Do not rush the onions. Medium heat, a wide heavy pot and a stir every few minutes. If they stick, add a splash of water and scrape the bottom. For more detail see our guide to caramelizing onions.
• Use good broth. Beef broth gives the classic taste. For a vegetarian soup, use a rich mushroom or vegetable broth and add a teaspoon of soy sauce for depth.
• Toast the bread first. Dry toast stays crisp under the cheese instead of dissolving into the soup.
• Use oven-safe bowls and place them on a baking sheet so they are easy to move and catch any drips.

Make ahead and storage

Make the soup up to 3 days ahead and keep it in the fridge; the flavor improves overnight. Reheat gently, then add the toast and cheese and broil just before serving. You can freeze the soup without the bread and cheese for up to 3 months.

What to serve with it

A crisp green salad and a glass of dry white wine are all you need. For a heartier dinner serve it with French dip sandwiches or follow it with a simple roast.

Frequently asked questions

Why is my onion soup bitter?

Usually the onions were cooked too hot and began to burn instead of caramelize. Lower the heat, add a splash of water when the pan looks dry and give them more time.

Can I make it without wine?

Yes. Replace the wine with extra broth and add a teaspoon of sherry vinegar or red wine vinegar at the end for acidity.

Which cheese is best?

Gruyère is traditional. Swiss, Comté or a mix of Gruyère and mozzarella also work; use cheese that melts and browns well.