French onion soup is a bistro classic built on one simple idea: onions, cooked slowly enough, become incredibly sweet and rich. Simmered in beef broth with thyme and a splash of wine, then crowned with toasted baguette and bubbling Gruyère, they make one of the most satisfying soups in the world.
From Paris markets to your kitchen
Onion soup has been eaten in France for centuries, but the version with cheese toast became famous in the 1800s in Les Halles, the old Paris food market, where workers and late-night diners warmed up with bowls of it before dawn.
Caramelizing onions takes time
There is no shortcut. The onions need 45 minutes to an hour over medium to medium-low heat, stirring now and then, to turn deep golden brown. Their natural sugars caramelize slowly, which creates the soup's sweet, complex flavor. Rushing over high heat burns the outside while the inside stays raw.
Which onions to use
Yellow onions are the best all-purpose choice: sweet when cooked and full of flavor. Adding one red onion or a sweet Vidalia gives extra depth. Slice them evenly, about 1/8 inch thick, so they cook at the same speed.
Scrape up the brown bits
The brown film that forms on the bottom of the pot, called fond, is concentrated flavor. When it builds up, add a splash of water and scrape it into the onions. The wine does the same job at the end.
A good broth matters
Since the soup has few ingredients, the broth makes a big difference. Use a good-quality beef broth or stock. Many cooks mix half beef and half chicken broth for a lighter, more balanced flavor.
The cheesy top
Gruyère melts beautifully and has a nutty, slightly sweet flavor. Comté, Swiss or a mix with Parmesan also work. Toast the bread first so it holds up in the broth, then pile on plenty of cheese so it covers the edges of the bowl.
No oven-safe bowls?
Place the toasts on a baking sheet, top with cheese and broil until bubbling, then float them on top of the soup in regular bowls.
Serving ideas
Serve with a green salad dressed with a mustard vinaigrette. For another rich, savory bowl, try our cream of mushroom soup or the hearty beef barley soup.
Storing and freezing
The soup without bread and cheese keeps for 5 days in the fridge and freezes for 3 months. Caramelized onions also freeze well, so you can make a double batch.
Use a heavy pot
A Dutch oven or other thick-bottomed pot spreads the heat evenly, so the onions brown slowly without scorching. A thin pot creates hot spots that burn the onions in one place while others stay pale.
How to slice onions without tears
Chill the onions for 30 minutes before slicing and use a very sharp knife, which crushes fewer cells and releases less of the gas that makes you cry. Cutting near a running range hood helps too.
Plan ahead
Caramelize the onions a day or two before, refrigerate them, and the soup comes together in 30 minutes on the night you serve it.
Frequently asked questions
Can I make French onion soup without wine?
Yes. Use extra broth and add a tablespoon of balsamic or sherry vinegar at the end for brightness.
Can I caramelize onions in a slow cooker?
Yes. Cook the sliced onions with the butter on low for 8 to 10 hours, then finish the soup on the stove.
Can I make it vegetarian?
Use a rich mushroom or vegetable broth and a splash of soy sauce for depth.
