What type of onion makes the best French onion soup?

Most cooks recommend yellow onions because they deliver a rich, slightly sweet, full-bodied flavor when caramelized. Many also combine yellow onions with sweet onions or even red onions to add layers of depth and complexity to the soup’s flavor.

Can I skip caramelizing the onions and cook everything directly in the slow cooker?

Yes — you can add raw onions straight into the slow cooker with butter or oil. However, the soup will taste far better if you take time to let the onions caramelize first; that slow, gentle browning brings out the natural sugars and creates the signature deep, sweet-savory onion flavor.

Is beef broth necessary — can I use vegetable or mushroom broth instead?

You don’t have to use beef broth. A good vegetable or mushroom broth works nicely for a vegetarian (or lighter) version and still gives a savory base. The result may be slightly different, but it can still be rich and comforting.

What cheeses melt best for the classic cheese-topped finish?

The traditional choice is Gruyère, which melts beautifully and develops a nutty, creamy crust. Swiss, Emmental, or a mix of cheeses (like adding a little mozzarella or Parmesan) also work — just aim for freshly shredded cheese for the best melting and flavor.

Slow Cooker French Onion Soup – Stunning Flavor Made Easy

Name: Slow Cooker French Onion Soup
Prep time: 20 min
Cook time: 8 h
Total time: 8 h 20 min
Yield: 6 servings
Cuisine: French
Ingredients: 4 large yellow onions, thinly sliced; 3 tablespoons unsalted butter; 1 tablespoon olive oil; 3 garlic cloves, minced; 1 teaspoon dried thyme or 2 teaspoons fresh thyme; 1 bay leaf; 1 teaspoon kosher salt; 1/2 teaspoon black pepper; 6 cups beef broth; 1 tablespoon Worcestershire sauce; 1 teaspoon sugar (optional, for caramelizing); 1 French baguette, sliced and toasted; 1 1/2 cups shredded Gruyère cheese; 1/2 cup shredded Swiss or Emmental cheese

Description

This easy slow cooker French onion soup recipe delivers deep, rich flavor with caramelized onions, savory broth, and a classic cheesy baguette topping for cozy weeknight dinners.

Ingredients

Scale

4 large yellow onions, thinly sliced
3 tablespoons unsalted butter
1 tablespoon olive oil
3 garlic cloves, minced
1 teaspoon dried thyme or 2 teaspoons fresh thyme
1 bay leaf
1 teaspoon kosher salt
1/2 teaspoon black pepper
6 cups beef broth
1 tablespoon Worcestershire sauce
1 teaspoon sugar (optional, for caramelizing)
1 French baguette, sliced and toasted
1 1/2 cups shredded Gruyère cheese
1/2 cup shredded Swiss or Emmental cheese

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Instructions

Slice the yellow onions into thin, even slices and mince the garlic.
Add the onions, garlic, butter, olive oil, thyme, sugar, salt, and pepper to the slow cooker and stir to coat the onions evenly.
Cook on low for 5 to 6 hours, stirring occasionally if possible, until the onions are very soft and deeply golden.
Pour in the beef broth and Worcestershire sauce, add the bay leaf, and cook on low for another 2 to 3 hours to develop rich flavor.
Toast the baguette slices and shred the Gruyère and Swiss or Emmental cheeses while the soup finishes cooking.
Ladle the hot soup into oven-safe bowls, top each with a toasted baguette slice, and cover generously with shredded cheese.
Place the bowls on a baking sheet and broil until the cheese is melted, bubbling, and lightly browned, then garnish with extra thyme if desired and serve immediately.

Notes

Caramelize the onions fully for the deepest, sweetest flavor in the soup.
Use low-sodium beef broth if you want more control over the salt level.
If the soup seems too thick after slow cooking, stir in a bit more broth or water before serving.
Always place the oven-safe bowls on a baking sheet before broiling to catch any bubbling cheese.
Cool leftovers completely before refrigerating or freezing, and add fresh bread and cheese when reheating for the best texture.

Prep Time: 20 minutes
Cook Time: 8 hours
Category: Soup
Method: Slow Cooker
Cuisine: French

Nutrition

Serving Size: 1 bowl (about 1 1/2 cups)
Calories: 420
Sugar: 7
Sodium: 1200
Fat: 20
Saturated Fat: 11
Unsaturated Fat: 8
Trans Fat: 0
Carbohydrates: 40
Fiber: 3
Protein: 18
Cholesterol: 55