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Slow-Cooker French Onion Soup

A hands-off take on classic French onion soup, featuring sweet caramelized onions, a splash of wine, and cheesy baguette topped with melted Gruyère.
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Prep Time 30 minutes
Cook Time 10 hours
Total Time 10 hours 30 minutes
Servings: 6 servings
Course: Appetizer, Soup
Cuisine: French
Calories: 350
Ingredients Method Notes

Ingredients
  

Main Ingredients
  • 4 large large onions, thinly sliced yellow or sweet onions work best
  • 4 tablespoons butter
  • 2 tablespoons olive oil
  • 4 cups beef broth substitute: mushroom or vegetable broth for vegetarian
  • 1 cup white wine dry; substitute: extra broth + 1 tbsp vinegar if you prefer no alcohol
  • 2 teaspoons thyme fresh or 1 tsp dried
  • to taste Salt
  • to taste Pepper
  • Baguette slices day-old baguette toasts nicely
  • 1 cup grated Gruyère cheese substitutes: Emmental, Comté, or a mix of Swiss cheeses

Method
 

Preparation
  1. Turn the slow cooker to low. Add the butter and olive oil so the fat melts and coats the insert.
  2. Add all the thinly sliced onions. Sprinkle in the thyme, salt, and pepper. Stir to combine.
Cooking
  1. Cover and cook on low for 8–10 hours. Stir once or twice if you remember; caramelization will occur slowly and evenly.
  2. When the onions are richly brown, stir in the beef broth and white wine.
  3. Replace the lid and let the soup cook on low for another hour to meld flavors.
Serving
  1. Preheat the oven broiler.
  2. Ladle soup into ovenproof bowls. Float a baguette slice on each and sprinkle with grated Gruyère.
  3. Place bowls under the broiler until the cheese is melted, golden, and bubbly. Watch closely — it takes 1–3 minutes.
  4. Carefully remove (bowls will be hot) and serve immediately.

Notes

For best results, use uniformly sliced onions and ensure to check at 7–8 hours if your slow cooker runs hot.