How To Make French Onion Soup in the Slow Cooker

updated Sep 30, 2020
How To Make French Onion Soup in the Slow Cooker
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(Image credit: Joe Lingeman)

When I discovered that you can make caramelized onions in the slow cooker with no stirring and virtually no work, it was only a small step from there to French onion soup. Although I’ve made this bistro favorite the classic, Julia-inspired way, now that I know how to make it in the slow cooker, I’m probably never going back to stovetop!

Here’s a step-by-step recipe for blissfully delicious French onion soup made from start to finish in the slow cooker. Well, right up until the cheesy toast. You still need the oven for that, but with soup this easy, what’s a little broiling at the end?

Why the Slow Cooker Makes the Best French Onion Soup

This is one of those magical recipes that makes you say Oh, can it be so easy? And the answer to that, for once, is yes. Yes it can.

(Image credit: Joe Lingeman)

Here’s how it goes: Throw a few pounds of sliced onions in the slow cooker overnight, or for 12 hours (you can slice them in your food processor if you really want to hurry this along). Cook until dark and jammy; you don’t even need to stir. Pour in some broth and walk away again. Come back at dinnertime, ladle out soup, and broil with cheese and toast.

Perfect French onion soup, absolutely hands-off. This is why slow cookers exist, you guys.

(Image credit: Joe Lingeman)

For Your Information

  • This recipe calls for three pounds of yellow onions and 10 cups of beef broth. Sweet onions are marvelous here if you can get them. Feel free to use a combination of yellow and red onions, if needed.
  • The onions cook alone for 12 hours. Opt for cooking overnight if you plan to serve the soup for dinner the next day. After adding the broth and seasoning to the soup, cook agin for another six to eight hours. The total cook time for this recipe is about 20 hours in the slow cooker.
  • You will smell this soup cooking. But it smells delicious, so it’s OK!
(Image credit: Joe Lingeman)

Key Steps for Slow Cooker French Onion Soup

  • Slice the onions into quarter-moons. You can use your mandoline, a food processor, or a chef’s knife to make quick work of this.
  • Cook the onions with butter and salt on low for 12 hours: This step is going to perfume the whole house. A reader suggested doing this step on a screened-in porch or in your garage if you need to avoid your home smelling like cooking onions for a whole day.
  • Add the broth and vinegar and cook for six to eight more hours on low: Again this can be done without supervision; there’s no stirring or checking.
  • Top with toasted bread and Gruyère and bake before serving: This final step really helps to thicken the soup. The baking allows for more evaporation and some of the starch from the bread softens and thickens the soup as well.

More on French Onion Soup

This is a pretty quick and basic guide to making French onion soup. If you want a little more detail, like some of my favorite flavoring add-ins (star anise, cinnamon, rosemary), my opinions on the right ratio of cheese to soup, and other notes on the classic preparation, visit our main French onion soup tutorial.

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(Image credit: Joe Lingeman)

How To Make French Onion Soup in the Slow Cooker

Serves 6 to 8

Nutritional Info


  • 3 pounds

    yellow onions, peeled, sliced, and cut into quarter-moons

  • 2 tablespoons

    unsalted butter, melted

  • 2 tablespoons

    olive oil

  • 2 teaspoons

    kosher salt, plus more as needed

  • Freshly ground black pepper

  • 10 cups

    reduced-sodium beef broth

  • 2 tablespoons

    balsamic vinegar

  • 3 tablespoons

    brandy (optional)

To Serve

  • 4 to 6

    toasted baguette slices per bowl

  • 1/3 cup

    grated Gruyère cheese per bowl (1 1/3 to 2 cups total)

  • Chopped shallot or fresh onion (optional)


  • Cutting board and chef's knife

  • 5-quart or larger slow cooker

  • Wooden spoon

  • Oven-safe soup bowls

  • Rimmed baking sheet


  1. Season the onions. Place the onions in a 5-quart or larger slow cooker. Stir in the butter, oil, salt, and a generous amount of pepper.

  2. Cook on LOW for 12 hours. Cover and cook on the LOW setting overnight until the onions should be dark golden-brown and soft, 12 hours or overnight.

  3. Add the broth and vinegar. Stir in the broth and vinegar.

  4. Cook for LOW 6 to 8 hours. Cover and continue cooking on the LOW setting for 6 to 8 hours. This is flexible; as long as your slow cooker holds moisture well (wrap a towel over the lid if quite a lot of steam escapes), you can cook the soup for hours. Longer cooking will only intensify the flavors. Taste and season with more salt and pepper if needed, and stir in the brandy if using.

  5. Portion the soup into oven-safe bowls. Arrange a rack in the upper third of the oven and heat to 350°F. Ladle the soup and onions into oven-safe soup bowls and place the bowls on a rimmed baking sheet.

  6. Top with toast and shredded cheese. Top each bowl with a slice of toast and a generous quantity of shredded Gruyère cheese, about 1/3 cup per bowl.

  7. Bake for 20 to 30 minutes. Bake until the cheese is completely melted, 20 to 30 minutes.

  8. Broil for 2 to 3 minutes. Turn the oven to broil. Broil until the cheese is bubbling and browned, 2 to 3 minutes. Remove from the oven and let cool for a few minutes. Serve with chopped fresh shallot or onion if desired.

Recipe Notes

Adjusting Consistency & Thickness: When I have made this, the soup has always been just the right consistency. But if yours seems a little thin or watery, you can quickly finish it off on the stovetop by simmering gently in a saucepan for 15 minutes or until the broth has reduced a little.

Nutrition Note: The nutrition info below does not include the bread and Gruyère for topping the soup.

This post has been updated, originally published February 2015.