Slow-Cooker French Onion Soup Recipe

There's nothing like a savory soup to satisfy cravings for a heart-warming meal. And when it comes to soups, French onion is high up on the list of deliciously decadent options. The humble onion takes center stage in the flavor profile, highlighted by a carb and cheese-laden finish. What more could you want? Well, recipe developer Erin Johnson takes an approach that maximizes flavor while reducing active cooking time. She shares, "This soup is my absolute favorite comfort food soup," describing it as "flavorful and rich." 

You'll need a slow-cooker for this French onion soup recipe, but that means the appliance will pretty much take care of the rest. A quick 5 minute prep time and at least 12 hours of simmering will produce caramelized onions like you've never had before. As Johnson remarks, "They turn out perfectly and require no effort." This is a great idea if you have guests over for dinner, since you can make the soup ahead of time and simply broil the cheese and bread garnish before service. Plus, "you can't go wrong with something topped with melted cheese," as our recipe developer points out. You'll be digging into this rich hearty soup with close to no hands-on work — but no one has to know that.

Get the ingredients for this slow-cooker French onion soup

The beauty of this recipe is in its simplicity, and that extends to the ingredients. You'll need to cut three large onions in half and slice them up, as well as a clove of garlic, minced. Measure out 3 tablespoons of butter, or Johnson indicates that a combination of 2 tablespoons of butter and 1 tablespoon of olive oil works as well. 

The rest of the broth consists of ½ teaspoon each of salt and pepper, 64 ounces of beef broth, 1 tablespoon each of Worcestershire sauce and balsamic vinegar, one bay leaf, and ½ teaspoon dried thyme. Finally, six slices of French bread and 1 cup of grated Gruyère cheese make up the soup topping.

Caramelize the onions with butter

Plug in your slow cooker and toss in the sliced onions, butter (or butter and olive oil), as well as ¼ teaspoon each of salt and pepper. Turn the setting to low and cook the onions overnight or between 6 and 8 hours. Johnson comments that "This is by far the easiest way to caramelize onions."

Cook the broth on low

Once you've cooked the onions for at least 6 hours, open up the slow cooker and pour in the beef broth. Then, season the contents with the minced garlic, Worcestershire sauce, balsamic vinegar, a bay leaf, dried thyme, and the rest of the salt and pepper to taste. Here, Johnson notes that "The exact amount of salt varies based on how salty your broth is." Instead of fussing over measurements, she recommends you "Let your taste be your guide."

Place the lid back on the slow cooker and cook the soup for another 6 to 8 hours on a low setting.

Assemble the soup

Once the onion broth is ready, it's time to finish assembling the soup. Carefully ladle the broth into ceramic bowls or ramekins that are heat proof. Then, place a slice of French bread on top and sprinkle cheese over it. 

This recipe makes six small portions, but if you want to use larger bowls for each serving, Johnson says, "you may want to add two pieces of bread to the top," and adds, "I like to completely cover the top with bread."

Broil the soup and serve

Turn on your oven's broiler setting. Once it's nice and hot, broil the soup for about 3 minutes until the cheese on top is golden brown and bubbly. Johnson warns, "Pay constant attention because it can burn quickly." You'll also want to be very careful when you are removing the ramekins from the oven as it will be piping hot. When it reaches a reasonable temperature, serve and enjoy this classic French onion soup.

Johnson notes that you can make the soup ahead of time and refrigerate it for up to three days or freeze it for up to three months. However, she instructs, "Just wait to top the soup with bread and cheese when you're ready to eat it."

Slow-Cooker French Onion Soup Recipe
5 from 37 ratings
This slow-cooker French onion soup provides a great hands-off approach to delicious homemade soup.
Prep Time
Cook Time
broiled French onion soup ramekin
Total time: 12 hours, 8 minutes
  • 3 large onions, halved and sliced
  • 3 tablespoons butter (or 2 tablespoons butter + 1 tablespoon olive oil)
  • ½ teaspoon salt, divided
  • ½ teaspoon pepper, divided
  • 64 ounces beef broth
  • 1 garlic clove, minced
  • 1 tablespoon Worcestershire sauce
  • 1 tablespoon balsamic vinegar
  • 1 bay leaf
  • ½ teaspoon dried thyme
  • 6 slices French bread
  • 1 cup grated Gruyère cheese
  1. Add the onions, butter, ¼ teaspoon salt, and ¼ teaspoon of pepper to the slow cooker.
  2. Cook on low overnight or for 6 to 8 hours.
  3. Add beef broth, garlic clove, Worcestershire sauce, balsamic vinegar, bay leaf, thyme, and the remainder of the salt and pepper to taste.
  4. Cook on low for an additional 6 to 8 hours.
  5. Ladle the broth into broiler-proof bowls or ramekins.
  6. Top with a slice of French bread and cheese.
  7. Broil until the cheese is brown and bubbly about 3 minutes.
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