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close up of broiled cheese and toast with fresh tyme sprig on a bowl of soup

Slow Cooker French Onion Soup

Bistro-style French Onion Soup made easy in the slow cooker!  You'll love how simple it is to make while you spoon up bites of sweet and savory onion, rich beef broth, hearty bread and gooey melted cheese.
Course Soup
Cuisine French
Keyword crock pot french onion soup, french onion soup, slow cooker french onion soup
Prep Time 10 minutes
Cook Time 18 hours
Total Time 18 hours 10 minutes
Servings 8 servings
Calories 426kcal
Author Lauren


  • 3 pounds sweet onions sliced in half-moons
  • 2 tablespoons unsalted butter melted
  • 2 tablespoons olive oil
  • ½ teaspoon salt
  • ¼ cup brandy
  • 8 cups beef broth
  • 4 sprigs of thyme
  • 2 bay leaves
  • ½ teaspoon chopped garlic
  • salt and pepper to taste
  • 1 loaf sourdough bread or another hearty bread like baguette or ciabatta
  • ½ pound Gruyere or Swiss cheese fresh grated


  • Caramelize the onions. Slice the onions into thin half-moon slices. In the slow cooker, combine the onions, butter, olive oil, and salt. Cook on low for 10 to 12 hours.
  • Make the soup. Once the onions have cooked down, add the brandy, beef broth fresh thyme, bay leaves, and garlic.
  • Cook the soup. Cook on low for 8 hours or high for 4 hours. Season with salt and black pepper to taste.
  • Toast and broil. Just before the soup is done cooking, slice the bread into 2 to 4 inch wide slices, and grate the cheese. Toast the bread until golden brown. Ladle the soup into oven-safe bowls, top with toast and grated cheese. Set all the bowls on a sheet tray and broil on high for 3 to 5 minutes until the cheese is melted and starting to brown. Serve immediately.


  • Onions can be caramelized in advance and then used to make the soup when you're ready.
  • There two options for timing to cook the soup.  Option 1: Cook the onions overnight for 10 to 12 hours.  In the morning, add the soup ingredients and cook on low for 8 hours or high for 4 hours.  Option 2:  Set the onions to cook in the morning all day, then make the soup and high cook for 4 hours for dinner.


Calories: 426kcal | Carbohydrates: 45g | Protein: 19g | Fat: 17g | Saturated Fat: 8g | Trans Fat: 1g | Cholesterol: 39mg | Sodium: 1438mg | Potassium: 432mg | Fiber: 3g | Sugar: 10g | Vitamin A: 383IU | Vitamin C: 9mg | Calcium: 363mg | Iron: 3mg