French Onion Soup in Slow-Cooker

French Onion Soup in a Slow-Cooker offers a convenient twist on the classic French dish, allowing the deep flavors of caramelized onions to develop slowly and effortlessly. This recipe transforms humble ingredients like onions, broth, and a touch of brandy into a rich, comforting soup enhanced with thyme and balsamic vinegar.

Topped with toasted baguette slices and a generous amount of melted Gruyere cheese, each bowl promises warmth and indulgence, perfect for cozy evenings or gatherings. Let your slow-cooker do the work overnight and throughout the day, filling your home with the inviting aroma of this beloved soup.

French Onion Soup in Slow-Cooker

Ingredients

3 pounds yellow onions, peeled, cut in half and thinly sliced

2 tablespoons unsalted butter, melted

2 tablespoons olive oil

2 ½ teaspoons kosher salt

½ teaspoon freshly ground black pepper, or more to taste

2 sprigs fresh thyme

¼ cup balsamic vinegar

8 cups (2 32-ounce cartons) beef broth

⅓ cup brandy, optional but recommended

4 to 6 baguette slices, toasted (1 slice per bowl)

1 ⅓ to 2 cups grated Gruyere cheese (⅓ cup per bowl)

Instructions:

Put the onion slices in a 5-quart (or larger) slow cooker, along with the butter, olive oil, salt, and black pepper.

Cover the crockpot and cook on low overnight, or for 10-12 hours. The onions should be dark golden brown and soft.

Add the beef broth, balsamic vinegar, and fresh thyme.

Cover and continue cooking on low for 6 to 8 hours.

Season to taste with additional salt and pepper, if desired, and stir in the brandy, if using.

Preheat the oven to 350 degrees Fahrenheit (180 degrees Celcius).

In evenly-portioned amounts, ladle the soup into oven-safe bowls. Place the bowls on a baking sheet to catch any spills while in the oven.

Top each bowl with a baguette slice and the shredded Gruyere.

Bake for 20 to 30 minutes, until the cheese is completely melted.

Turn the oven to broil and brown the tops for 2 to 3 minutes, or until the cheese is bubbling and browned. Remove from the oven and let cool for a few minutes before enjoying!

Notes:

Slow-Cooker Convenience: Using a slow-cooker allows the onions to caramelize slowly and develop deep flavors without constant attention. This method is ideal for busy schedules or when you want to prepare ahead.

Ingredient Preparation: Slicing the onions thinly ensures they cook evenly and develop a rich, sweet flavor as they caramelize. The addition of butter and olive oil helps to coat and enhance the onions during cooking.

Flavor Enhancers: Fresh thyme and balsamic vinegar add layers of flavor to the soup, complementing the sweetness of the caramelized onions and the richness of the beef broth.

Optional Brandy: The addition of brandy is optional but recommended as it adds a subtle depth and complexity to the soup. It can be omitted for a non-alcoholic version.

Toasting Baguette Slices: Toasting the baguette slices before adding them to the soup helps them maintain their texture when submerged, preventing them from becoming too soggy.

Cheese Topping: Gruyere cheese, known for its nutty and slightly sweet flavor, melts beautifully and forms a delicious golden crust when baked. Adjust the amount based on your preference for cheesiness.

Baking and Broiling: Baking the soup with the cheese-topped baguette slices allows the flavors to meld together while creating a comforting crust. Broiling briefly at the end ensures a bubbly, golden-brown cheese topping.

Serving Tips: Let the soup cool slightly before serving to avoid burns from the hot bowls. The combination of flavors and textures makes this soup a satisfying main course or appetizer, perfect for colder days.

Make-Ahead and Freezing: This soup can be prepared in advance and freezes well. Reheat gently on the stovetop or in the microwave, adding fresh baguette and cheese before serving.

Variations: Experiment with different types of cheese or herbs for a personalized touch. Some variations include adding a splash of Worcestershire sauce or substituting beef broth with vegetable broth for a vegetarian version.

Nutrition Information:

Frequently Asked Questions:

Can I use other types of onions for this soup?

While yellow onions are traditional for French onion soup due to their sweet flavor when caramelized, you can experiment with other varieties like red onions or sweet onions for slightly different flavor profiles.

How do I store leftover French onion soup?

Let the soup cool completely, then transfer it to airtight containers.

Store in the refrigerator for up to 3-4 days or freeze for up to 3 months.

Can I skip the brandy in this recipe?

Yes, the brandy is optional. It adds depth of flavor, but the soup will still be delicious without it.

What if I don’t have a slow cooker?

You can make this soup on the stovetop by caramelizing the onions in a large pot over low heat, then adding the remaining ingredients and simmering for 1-2 hours until the flavors meld.

How can I make this soup vegetarian?

Substitute vegetable broth for the beef broth and omit the bacon.

You can also add a tablespoon of soy sauce or miso paste to enhance the umami flavor.

Can I use dried thyme instead of fresh?

Yes, you can use dried thyme if you don’t have fresh.

Use about 1 teaspoon of dried thyme instead of the fresh sprigs.

What’s the best way to toast the baguette slices?

Preheat your oven to 350°F (180°C) and arrange the baguette slices on a baking sheet.

Bake for about 5-7 minutes, flipping halfway through, until they are golden and crispy.

How do I know when the onions are caramelized enough?

Caramelized onions should be dark golden brown and very soft.

They should have a sweet, rich aroma and a slightly sticky texture.

Can I use a different type of cheese instead of Gruyere?

Gruyere cheese is traditional for French onion soup due to its nutty flavor and melting properties.

However, you can substitute with Swiss cheese or a combination of Swiss and Parmesan.

Can I make this soup ahead of time?

Yes, French onion soup actually improves in flavor when made ahead.

Prepare the soup as instructed, cool it down, and refrigerate.

Reheat gently on the stovetop or in the oven before adding the bread and cheese toppings.