- 1/2 cup unsalted butter, melted
- 2 tablespoons extra-virgin olive oil
- 4 pounds yellow onion, peeled and sliced
- 10 cups beef broth
- 1 bay leaf
- Salt and pepper, to taste
- 1/4 cup all-purpose flour
- 2 tablespoons balsamic vinegar
- 1 tablespoon Worcestershire sauce
- 1 baguette, sliced diagonally and toasted
- 1 1/2 cups Gruyere or swiss cheese, grated or sliced
- Minced chives, for garnish
InstructionsPrint This Recipe
- Add butter, olive oil, and onions to a large skillet over medium heat. Cook until onions start to caramelize.
- Pour onions into the slow cooker and top with beef broth, bay leaf, salt, and pepper.
- In a small bowl whisk together flour, balsamic vinegar, and Worcestershire sauce until smooth. Stir into soup.
- Cover and cook on low for 6 to 8 hours, or on high for 3 to 4 hours.
- Scoop soup into oven-safe bowls. Top with 1 slice of toasted bread and cheese. Broil for 5 minutes, or until cheese is melted and bubbly.
- Top with chives, serve and enjoy!