You're going to love the taste of this Slow Cooker french Onion Soup. Loaded with caramelized onions and Gruyere cheese. One bowl just isn't going to be enough!
Place the sliced onions in a 5 qt. or larger slow cooker.
Drizzle over the butter and olive oil.
Sprinkle the salt and pepper over the top and cook on LOW for 12 hours.
Add in the broth and balsamic vinegar.
Stir and cook on LOW for an additional 8 hours.
Preheat oven to 350F.
Arrange 4-6 oven safe bowls on a sturdy baking sheet.
Ladle the soup into the bowls.
Top with a slice of toasted french bread and 1/3 cup of cheese each. Add more cheese if you like.
Bake 20-25 minutes or until the cheese is browned to your liking.
Cool 2-3 minutes before serving.