Go Back

Slow Cooker French Onion Soup

Elina James
Print Recipe Pin Recipe
Prep Time 15 minutes
Cook Time 8 hours
Total Time 8 hours 15 minutes
Course Soups
Cuisine French
Servings 6 servings
Calories 300 kcal

Ingredients
  

  • For the Soup:
  • 6 large yellow onions, thinly sliced
  • 3 tbsp unsalted butter
  • 1 tbsp olive oil
  • 1 tsp salt
  • 1 tsp sugar (optional, helps caramelization)
  • 4 cloves garlic, minced
  • 2 tsp fresh thyme leaves (or 1 tsp dried thyme)
  • 2 bay leaves
  • ½ cup dry white wine (or red wine; optional)
  • 6 cups beef broth (or vegetable broth for vegetarian)
  • 1 tbsp balsamic vinegar (optional, for depth)
  • Salt and pepper, to taste
  • For the Topping:
  • 1 baguette, sliced into 1-inch thick rounds
  • 2 cups shredded Gruyère cheese (or Swiss, Comté, or mozzarella)
  • ½ cup Parmesan cheese (optional, for extra flavor)

Instructions
 

  • Step 1: Prepare the Onions : Peel and thinly slice the onions (about ⅛-inch thick). In a large skillet over medium heat, melt butter with olive oil. Add sliced onions, salt, and sugar; cook for 10–15 minutes, stirring often, until softened and golden brown. (Optional step — you can skip this sautéing and let onions caramelize directly in the slow cooker, but pre-caramelizing enhances flavor.)
  • Step 2: Transfer to Slow Cooker : Place caramelized onions into the slow cooker. Add garlic, thyme, bay leaves, wine (if using), broth, and balsamic vinegar. Stir gently to combine.
  • Step 3: Slow Cook : Cover and cook on low for 8 hours or high for 4 hours, until onions are tender and flavors meld beautifully. Remove bay leaves before serving. Taste and adjust salt and pepper.
  • Step 4: Prepare the Bread u0026 Cheese : Preheat the oven to 400°F (200°C). Place baguette slices on a baking sheet and toast for 8–10 minutes until golden.
  • Step 5: Assemble u0026 Broil : Ladle hot soup into oven-safe bowls. Place one or two toasted baguette slices on top of each bowl. Sprinkle generously with Gruyère and Parmesan cheese. Place bowls on a baking sheet and broil for 2–4 minutes, until the cheese is melted and bubbly with golden edges.
  • Step 6: Serve : Carefully remove from the oven (bowls will be hot!). Serve immediately, garnished with extra thyme if desired.