Winter is all about getting and staying cozy. Even if you’re a winter outdoors enthusiast, there’s nothing better than coming up to a tasty, gut-warming soup. This Slow Cooker French Onion Soup recipe, courtesy of America’s Test Kitchen, is packed with flavor!

Slow Cooker French Onion Soup
Slow Cooker French Onion Soup
Photo: Steve Klise, courtesy of America’s Test Kitchen.

Why this recipe works: It can be a hassle to stand over a pot of caramelized onions when a craving for this French classic strikes. We looked to the slow cooker for simplification. Replicating the meaty flavor of the soup was more of a challenge, as the slow, long cooking can result in washed-out flavor. We found that soy sauce, sherry, and thyme added early on helped boost flavor and the addition of beef bones to store-bought chicken and beef broths reproduced the rich meatiness of the classic. Apple butter highlighted the flavor of the onions without drawing attention to itself and also helped make for a rich, silky broth.


For the soup
    • 2 pounds beef bones (stocked in the frozen foods aisle of most supermarkets)
    • 4 tablespoons unsalted butter
    • 4 pounds yellow onions, halved and sliced through root end into ¼-inch-thick slices*
    • Salt and pepper
    • 1 tablespoon packed brown sugar
    • 1 teaspoon minced fresh thyme
    • ¾ cup apple butter
    • ¾ cup dry sherry
    • 5 tablespoons all-purpose flour
    • ¼ cup soy sauce
    • 2 cups chicken broth
    • 2 cups beef broth
For the cheese croutons
    • 1 small baguette, cut into ½-inch slices
    • 10 ounces Gruyère cheese, shredded (2½ cups)


For the soup
    1. Arrange beef bones on paper towel-lined plate. Microwave until well browned, 8 to 10 minutes. Meanwhile, set slow cooker to high. Add butter, cover, and cook until melted. Add onions, 2 teaspoons salt, 1 teaspoon pepper, brown sugar, and thyme. Stir apple butter, sherry, flour, and soy sauce together in small bowl until smooth. Pour over onions and toss to coat. Tuck bones under onions around edge of slow cooker. Cover and cook on high heat until onions are softened and deep golden brown, 10 to 12 hours (start checking onions after 8 hours). Note: Cooked onions can be refrigerated for up to 1 day.
    2. Remove bones from slow cooker. Heat broths in microwave until beginning to boil. Stir into slow cooker. Season with salt and pepper to taste.
For the cheese croutons
    1. Adjust oven rack to upper-middle position (about 6 inches from broiler element) and heat oven to 400 degrees. Arrange bread slices in single layer on baking sheet and bake until bread is golden at edges, about 10 minutes. Heat broiler. Divide cheese evenly among croutons and broil until melted and bubbly, 3 to 5 minutes.
    2. Ladle soup into bowls and top each with 2 croutons. Serve.

*Note: After halving the onions, slice them through the root end for hearty slices that will hold up to long cooking.

Complete Slow Cooker book

Want more slow cooker recipes from America’s Test Kitchen?
Check out The Complete Slow Cooker Cookbook… it’s got 400 of them!



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