Description
This Slow Cooker French Onion Soup recipe offers a rich, deeply caramelized onion flavor layered with savory beef broth, enhanced by balsamic vinegar and cognac, topped with toasted baguette slices and melted Gruyère cheese. Perfect for a comforting meal, it utilizes slow cooking to develop intense flavors with minimal hands-on time.
Ingredients
Scale
Soup Ingredients
- 3 pounds yellow onions, peeled, sliced, and cut into quarter-moons (about 4 large onions)
- 2 tablespoons unsalted butter, melted
- 2 tablespoons olive oil
- 2 teaspoons kosher or sea salt, plus more as needed
- Freshly ground black pepper (optional)
- 8 cups (64 ounces) reduced-sodium beef broth
- 2 tablespoons balsamic vinegar
- 3 tablespoons cognac or brandy (optional)
Topping Ingredients
- Baguette, cut into 1-inch slices (2-3 slices per bowl)
- 1/3 cup grated Gruyère cheese per bowl (about 2 cups total for 6 bowls)
Instructions
- Caramelize Onions: Add the sliced onions, olive oil, melted butter, salt, and pepper (if using) into the slow cooker. Cook on low for about 14 hours, stirring 2-3 times if possible, until the onions are deeply caramelized and golden brown.
- Prepare Soup Base: Once the onions are caramelized, add the reduced-sodium beef broth, balsamic vinegar, and cognac (if using). Stir well, then cook on low for an additional 7-8 hours to blend the flavors and develop a rich soup.
- Toast Baguette Slices: Preheat the oven to 425°F (220°C). Slice the baguette into 1-inch thick pieces, arranging 2-3 slices per bowl depending on bowl size. Place on a sheet pan and bake for about 7-8 minutes until the bread becomes crisp and toasted.
- Assemble and Bake Soup Bowls: Ladle the hot onion soup into oven-safe bowls. Dunk 2-3 toasted baguette slices into the soup and arrange on top. Generously sprinkle each bowl with 1/3 cup grated Gruyère cheese. Place the bowls on a sheet pan for easy handling and transfer to the oven. Bake for about 10 minutes until the cheese melts and becomes bubbly and gooey. Serve cautiously as bowls will be hot.
Notes
- Cooking times may vary slightly depending on the slow cooker model.
- Stirring occasionally during onion caramelization helps prevent burning and ensures even cooking.
- Using reduced-sodium broth helps control the saltiness of the soup; adjust salt to taste when serving.
- Cognac or brandy is optional but adds a wonderful depth of flavor to the soup.
- Gruyère cheese is traditional for French onion soup, but Swiss or a mix of shredded cheeses can be substituted if unavailable.
- Ensure to use oven-safe bowls when melting the cheese topping to avoid damage or accidents.
