French Onion Soup with Meatballs

In a 4-qt. slow cooker, combine the first 9 ingredients. Cook, covered, on low until onions are tender, 8-10 hours., Ladle soup into 6 broiler-safe 16-oz. ramekins. Top each with 3 slices of bread and 2 slices of cheese. Broil 4-6 in. from heat until cheese is melted, 2-3 minutes. Serve immediately.