Slow Cooker French Onion Soup

Step 1
Heat butter in a large, heavy pot over medium-high heat. Add onions; cook and stir until translucent, about 10 minutes. Sprinkle onions with sugar; reduce heat to medium. Cook, stirring constantly, until onions are soft and caramelized, at least 30 minutes. Stir in garlic and cook until fragrant, about 1 minute.

Step 2
Stir sherry into onion mixture; scrape the bottom to dissolve the small bits of browned food. Transfer onions into a slow cooker and pour in beef broth. Season with sea salt; stir in thyme and bay leaf. Cover and cook on Low for 8 to 10 hours or on High for 4 to 6 hours.

Step 3
Preheat the oven broiler about 10 minutes before serving. Set an oven rack about 8 inches from the heat source.

Step 4
Arrange bread slices on a baking sheet; broil until toasted, 1 to 2 minutes per side.

Step 5
Lightly toss Gruyere, Emmental, Parmesan, and mozzarella cheeses in a bowl. Fill oven-safe soup crocks 3/4 full with onion soup and float a bread slice in each bowl. Top each with about 2 tablespoons cheese mixture.

Step 6
Place filled bowls onto a baking sheet and broil until cheese topping is lightly browned and bubbling, about 2 minutes.