Slow Cooker French Onion Soup

french onion soup with fresh bread in a crock pot.

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With a caramelized onion and beef stock base, this slow cooker French onion soup is deeply savory, shockingly simple, and 100% gourmet.

Prep Time15

Cook Time6 45

Additional Time10

Total Time7 10

Course: Soup

Cuisine: French

Keyword: slow cooker french onion soup

Servings:

Author: Lisa Bass

Equipment

1 Slow Cooker

Ingredients

2 tablespoons butter
3 pounds of sweet onions
¾ cup red wine like merlot
6 cups beef broth
½ tablespoon fresh thyme leaves or 1 tsp dried
3 bay leaves
2 tablespoons all-purpose flour
½ teaspoon balsamic vinegar

For the topping

3 tablespoons olive oil
1 French baguette
250 grams Gruyère cheese

Instructions

Pour two tablespoons of melted butter into the bottom of a slow cooker. Add 3 pounds of thinly sliced sweet onions, stirring to coat evenly in the butter.
Cover and cook on high for 5 – 6 hours, stirring occasionally, until the onions are deeply caramelized and golden brown.
Add beef broth, red wine, fresh thyme, and bay leaves. Stir well, cover, and cook on high for 30 – 45 minutes.
In a small bowl, whisk 2 tablespoons of all-purpose flour with about ½ cup of hot broth from the crockpot until smooth. Stir the flour mixture back into the soup to slightly thicken.
Add ½ teaspoon of balsamic vinegar for acidity. Taste and season with salt and freshly ground black pepper. The soup should be well-seasoned and slightly salty.
Preheat the broiler. Slice the French baguette into ½-inch thick slices and place on a parchment-lined baking sheet. Brush both sides with olive oil. Broil on the top rack for 2 – 3 minutes per side, or until golden brown — watch closely to avoid burning.
Remove the bay leaves and ladle hot soup into oven-safe bowls or crocks. Top each bowl with 2 – 3 toasted baguette slices, then cover generously with grated Gruyère cheese.
Place bowls on the baking sheet and broil for 1 – 2 minutes or until the cheese is melted and bubbly. Carefully remove from the oven (the bowls will be hot!) and serve immediately with a few sprigs of fresh thyme on top for garnish.

Notes

Please double-check amounts when using the multiplying feature in the recipe card.

Slice your onions very thinly.

Toast extra baguette slices to add more at the bottom of your bowls.

Stir a few times during caramelizing. While the crockpot is low-maintenance, giving the onions a quick stir every 1 – 2 hours helps prevent burning around the edges and promotes even browning.

Use a wine you'd enjoy drinking.

Use sweet onions for the best caramelization. Varieties like Vidalia or Walla Walla give a mellow, rich flavor without bitterness.

You can caramelize the onions a day in advance and refrigerate them—just add broth and finish the next day.

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