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- 4 pounds yellow onions, peeled and chopped
- 10 cups beef broth
- 1 baguette, sliced diagonally and toasted
- 1 bay leaf
- 1 1/2 cups Gruyere or swiss cheese, grated or sliced
- 1/2 cup (1 stick) unsalted butter, melted
- 1/4 cup all-purpose flour
- 2 tablespoons extra-virgin olive oil
- 2 tablespoons balsamic vinegar
- 1 tablespoon Worcestershire sauce
- kosher salt and freshly ground pepper, to taste
- Minced chives, for garnish
- Place chopped onions, butter, and olive oil in a large skillet or pot, and cook over medium heat for 10-15 minutes, or until onions start to caramelize.
- In a small bowl, whisk flour into balsamic vinegar and Worcestershire sauce and mix until smooth.
- Transfer onions to slow cooker. Pour in beef broth, bay leaf, salt, pepper, and flour mixture, and stir well to combine.
- Cover and cook on low for 6-8 hours, or on high for 3-4 hours.
- Ladle soup into oven proof bowls, top each with 1 slice of bread and cheese, and broil, about 5 minutes or until cheese is melted and bubbly.
- Garnish with chives, serve hot and enjoy!