Slow Cooker French Onion Soup
3 pounds yellow onions, peeled and sliced
2 tablespoons olive oil
1 teaspoon salt, plus more to taste
Freshly ground black pepper
2 tablespoons balsamic vinegar
10 cups beef bone broth or beef broth
3 tablespoons Sherry, optional
For assembling the soup:
6 baguette slices for each bowl
1 1/3 to 2 cups grated or shaved Gruyere cheese
Chopped parsley for garnishing
Place onions in a 6-quart slow cooker. Stir in butter and olive oil. Season with salt and pepper. Cover and cook on low for 12 hours.
Add balsamic vinegar and beef bone broth into the slow cooker. Add Sherry if using.
Cover and cook on low for another 6-8 hours.
Preheat oven to 350°F.
Portion and ladle the soup and onions into oven-safe bowls and place the bowls on a baking sheet.
Top each bowl with a slice of toast and a generous amount of grated or shaved Gruyere cheese.
Bake on a rack in the upper third of the oven for 20-30 minutes or until the cheese is melted.
Switch oven to broil and broil the soup for 2-3 minutes until the cheese is browned.
Garnish with chopped parsley and serve warm.