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Close-up of slow cooker French onion soup with toasted bread and melted, bubbly cheese in a white bowl, garnished with thyme.

Slow Cooker French Onion Soup

This slow cooker French onion soup is a rich, flavorful classic made effortlessly with caramelized onions, savory broth, crispy bread, and gooey cheese. Perfect for chilly nights or entertaining, this dish offers restaurant-quality taste without the hassle. Customize with toppings and enjoy the best slow cooker onion soup experience!
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Prep Time 30 minutes
Cook Time 8 hours
Total Time 8 hours 30 minutes
Servings: 8 servings
Course: Soup
Cuisine: French
Calories: 310
Ingredients Equipment Method Notes

Ingredients
  

  • 6 tablespoons butter
  • 4 large yellow onions sliced and separated into rings
  • 1 tablespoon white sugar
  • 2 cloves garlic minced
  • 1/2 cup cooking sherry
  • 7 cups reduced-sodium beef broth
  • 1 teaspoon sea salt or to taste
  • 1/4 teaspoon dried thyme
  • 1 bay leaf
  • 8 slices French bread
  • 1/2 cup shredded Gruyere cheese
  • 1/3 cup shredded Emmental cheese
  • 1/4 cup freshly shredded Parmesan cheese
  • 2 tablespoons shredded mozzarella cheese

Equipment

  • Large heavy pot
  • Slow Cooker
  • Baking sheet
  • Oven-safe bowls

Method
 

  1. Heat butter in a large, heavy pot over medium-high heat. Add sliced onions and cook, stirring, until translucent (about 10 minutes). Sprinkle on the sugar and reduce heat to medium. Stir constantly until the onions are soft, golden, and caramelized, about 30 minutes. Stir in minced garlic and cook for one more minute until fragrant.
  2. Pour cooking sherry into the pot and scrape up any flavorful browned bits stuck to the bottom.
  3. Add the onion mixture to your slow cooker. Pour in the beef broth, then sprinkle in the sea salt, dried thyme, and bay leaf. Stir everything together.
  4. Cover the slow cooker and cook on low for 8-10 hours or high for 4-6 hours.
  5. When the soup is ready, preheat your oven broiler. Arrange sliced bread on a baking sheet and toast each side for 1-2 minutes until golden.
  6. Ladle soup into oven-safe bowls, leaving room at the top. Place a slice of toasted bread on each portion, then sprinkle generously with the cheese mixture (Gruyere, Emmental, Parmesan, and mozzarella).
  7. Broil the bowls until the cheese is melted, bubbly, and lightly browned, about 2 minutes. Remove carefully, serve hot, and enjoy!

Notes

For a quick shortcut, you can skip caramelizing the onions on the stovetop. Simply melt the butter directly in your crockpot, toss in the onions and sugar, and let them cook until soft. While the flavor won’t be as rich, it’s a handy time-saver! Feel free to let guests assemble their own servings with bread and cheese for a customizable experience.