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My father judged every restaurant he ever walked into by its French onion soup, and he told every waiter the same thing: my wife’s is better. He was right, and the reason was patience. My mother cooked the onions for the better part of an hour, a whole pot of them in butter over a low flame, stirred now and then with a wooden spoon, until they went from white to gold to a deep, dark brown and tasted sweet as candy. Then rich beef broth, a bay leaf and some thyme, toasted bread floated on top, and a thick blanket of Swiss and Gruyere melted under the broiler until it bubbled over the sides of the crock.
Everything good about this soup comes from the onions. The color of the onions is the color of the soup, and the sweetness of the onions is its flavor. The trouble is that onions cooked slowly can also stick and scorch, and burnt onions make a bitter soup. Here is how to get the onions deep brown without burning them, using steady medium-low heat, a heavy pot, a little salt, and a splash of water whenever the bottom starts to darken too fast.
Why this French onion soup works
- A heavy pot. A Dutch oven or heavy soup pot spreads the heat so the onions brown evenly instead of burning in spots.
- Salt at the start. Salt draws water out of the onions so they soften and collapse quickly before they start to brown.
- Low and slow. Forty-five minutes to an hour turns the natural sugar in the onions into deep caramel.
- Water to deglaze. A spoonful of water lifts the brown layer off the bottom of the pot and puts it back into the onions.
- Good beef broth. The broth carries the soup, so use the richest you can find, or homemade.
Ingredients
Makes about 6 bowls. You need oven-safe soup crocks or bowls for the cheese lid. The measurements are in US cups with grams in brackets.
For the soup
- 4 tablespoons unsalted butter (55g)
- 1 tablespoon vegetable oil
- 3 pounds yellow onions, about 6 large, halved and thinly sliced (1.4kg)
- 1 teaspoon fine salt, plus more to taste
- 1/2 teaspoon granulated sugar (optional, helps on cold onions)
- 2 tablespoons all-purpose flour
- 8 cups rich beef broth (2 liters)
- 2 bay leaves
- 4 sprigs fresh thyme, or 1/2 teaspoon dried thyme
- 1/2 teaspoon black pepper
- 1 teaspoon cider vinegar, to finish
For the cheese lid
- 1 small baguette, cut into 12 slices about 3/4 inch thick
- 1 1/2 cups shredded Gruyere (150g)
- 1 cup shredded Swiss cheese (100g)
Plain yellow onions give the deepest flavor. Sweet onions work but brown faster and make a sweeter soup. Slice them evenly, a little less than 1/4 inch, so they cook at the same speed.
How to make French onion soup
- Melt the butter with the oil in a large heavy pot or Dutch oven over medium heat. Add all the onions and the salt and stir to coat them. The pot will be very full; they shrink a lot.
- Cover the pot and cook for 10 minutes, stirring twice, until the onions have softened and given up their juice.
- Take the lid off and turn the heat to medium-low. Add the sugar if using. Cook, stirring every 5 minutes or so, for 45 to 60 minutes. The onions will go from pale to golden, then to a deep chestnut brown.
- Watch the bottom of the pot. Whenever a brown layer forms and starts to darken quickly, add 2 tablespoons of water and scrape the bottom with a wooden spoon. The water lifts the brown bits back into the onions. Do this as many times as you need.
- When the onions are a deep, even brown and jammy, sprinkle in the flour and stir for 2 minutes.
- Pour in the beef broth slowly, scraping the bottom of the pot well. Add the bay leaves, thyme and pepper.
- Bring to a boil, then lower the heat and simmer uncovered for 25 to 30 minutes. Take out the bay leaves and thyme stems. Stir in the vinegar, then taste and add salt and pepper as needed.
- While the soup simmers, heat the oven to 400 degrees (200°C) and toast the baguette slices on a baking sheet for 8 to 10 minutes, turning once, until dry and crisp.
- Turn the oven to broil. Set 6 oven-safe crocks on a rimmed baking sheet and ladle in the hot soup.
- Float 2 slices of toast on each bowl and pile the Gruyere and Swiss on top, right out to the edges so it touches the rims.
- Broil 6 inches from the heat for 3 to 5 minutes, until the cheese is bubbling and browned in spots. Let the bowls sit for 2 minutes before serving. They are very hot.
Tips that make the difference
- Do not turn the heat up to hurry the onions. High heat browns the outside of the slices before the sugar inside has caramelized, and it burns them.
- Cut the onions from root to tip, not in rings. They hold their shape in the soup instead of turning to mush.
- Dry toast is the secret to a lid that holds. Soft bread sinks and disappears into the broth.
- No crocks? Ladle the soup into bowls and top with cheese toasts made under the broiler on a baking sheet.
Variations
- Make-ahead onions. Caramelize the onions a day ahead and refrigerate them. The soup comes together in 30 minutes.
- Slow cooker. Cook the sliced onions with the butter and salt on low for 10 hours with the lid slightly open, then finish on the stove.
- Different cheese. Provolone or mozzarella melts beautifully if you cannot find Gruyere.
- Lighter. Skip the bread and cheese and serve the soup with a grilled cheese sandwich on the side.
Storage and make-ahead
The soup, without the bread and cheese, keeps in the refrigerator for up to 4 days and tastes even better on the second day.
Freeze the soup for up to 3 months. Thaw overnight in the refrigerator and reheat gently.
Always add fresh toast and cheese when you reheat it.
Frequently asked questions
Why is my French onion soup bitter?
The onions were cooked on heat that was too high and some of them burned. Keep the heat medium-low and add a splash of water whenever the bottom of the pot darkens too fast.
How long does it take to caramelize onions?
About 45 minutes to an hour after they soften. There is no shortcut that gives the same deep flavor.
What cheese is used on French onion soup?
Gruyere is traditional, and Swiss cheese is the diner classic. A mix of the two gives the best melt and flavor.
Can I use chicken broth?
Beef broth gives the classic dark, rich soup. Chicken broth makes a lighter, sweeter soup that is still good.
Give the onions their hour, pile on the cheese, and then go ahead and judge every restaurant by it, the way my father did.
French Onion Soup (Deep Brown Onions, Beef Broth, Bubbling Cheese Lid)
Classic French onion soup made with onions slowly caramelized in butter until deep brown, simmered in rich beef broth with thyme and bay, and finished under the broiler with toasted bread and melted Swiss and Gruyere.
Ingredients
- 4 tablespoons unsalted butter
- 1 tablespoon vegetable oil
- 3 pounds yellow onions, about 6 large, halved and thinly sliced
- 1 teaspoon fine salt, plus more to taste
- 1/2 teaspoon granulated sugar
- 2 tablespoons all-purpose flour
- 8 cups rich beef broth
- 2 bay leaves
- 4 sprigs fresh thyme, or 1/2 teaspoon dried thyme
- 1/2 teaspoon black pepper
- 1 teaspoon cider vinegar, to finish
- 1 small baguette, cut into 12 slices about 3/4 inch thick
- 1 1/2 cups shredded Gruyere
- 1 cup shredded Swiss cheese
Instructions
- Melt the butter with the oil in a large heavy pot or Dutch oven over medium heat. Add all the onions and the salt and stir to coat them. The pot will be very full; they shrink a lot.
- Cover the pot and cook for 10 minutes, stirring twice, until the onions have softened and given up their juice.
- Take the lid off and turn the heat to medium-low. Add the sugar if using. Cook, stirring every 5 minutes or so, for 45 to 60 minutes. The onions will go from pale to golden, then to a deep chestnut brown.
- Watch the bottom of the pot. Whenever a brown layer forms and starts to darken quickly, add 2 tablespoons of water and scrape the bottom with a wooden spoon. The water lifts the brown bits back into the onions. Do this as many times as you need.
- When the onions are a deep, even brown and jammy, sprinkle in the flour and stir for 2 minutes.
- Pour in the beef broth slowly, scraping the bottom of the pot well. Add the bay leaves, thyme and pepper.
- Bring to a boil, then lower the heat and simmer uncovered for 25 to 30 minutes. Take out the bay leaves and thyme stems. Stir in the vinegar, then taste and add salt and pepper as needed.
- While the soup simmers, heat the oven to 400 degrees (200°C) and toast the baguette slices on a baking sheet for 8 to 10 minutes, turning once, until dry and crisp.
- Turn the oven to broil. Set 6 oven-safe crocks on a rimmed baking sheet and ladle in the hot soup.
- Float 2 slices of toast on each bowl and pile the Gruyere and Swiss on top, right out to the edges so it touches the rims.
- Broil 6 inches from the heat for 3 to 5 minutes, until the cheese is bubbling and browned in spots. Let the bowls sit for 2 minutes before serving. They are very hot.
