Direction

1. Gather all ingredients.
2. Heat butter in a large, heavy pot over medium-high heat. Add onions; cook and stir until translucent, about 10 minutes.
3. 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.
4. Stir sherry into onion mixture; scrape the bottom to dissolve the small bits of browned food.
5. Transfer onions into a slow cooker and pour in beef broth. Season with sea salt; stir in thyme and bay leaf.
6. Cover and cook on Low for 8 to 10 hours or on High for 4 to 6 hours.
7. Preheat the oven broiler about 10 minutes before serving. Set an oven rack about 8 inches from the heat source. Arrange bread slices on a baking sheet; broil until toasted, 1 to 2 minutes per side.
8. Lightly toss Gruyere, Emmental, Parmesan, and mozzarella cheeses in a bowl.
9. Fill oven-safe soup crocks 3/4 full with onion soup and float a bread slice in each bowl.
10. Top each with about 2 tablespoons cheese mixture.
11. Place filled bowls onto a baking sheet and broil until cheese topping is lightly browned and bubbling, about 2 minutes.