Broiled French Onion Soup with Cheesy Toasted Bread

Gather all ingredients.

In a large pot, heat butter over medium-high heat. Add onions and cook until translucent, about 10 minutes.

Sprinkle sugar over onions; reduce heat to medium. Cook for 30 minutes, stirring constantly, until onions are soft and caramelized.

Stir in garlic and cook until fragrant, about 1 minute.

Add sherry to the onion mixture, scraping the bottom to dissolve any browned bits.

Transfer onions to a slow cooker, pour in beef broth, and 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.

Preheat oven's broiler about 10 minutes before serving. Position an oven rack 8 inches from the heat source.

Toast bread slices on a baking sheet under the broiler for 1 to 2 minutes per side.

Combine Gruyere, Emmental, Parmesan, and mozzarella cheeses in a bowl.

Fill oven-safe soup crocks 3/4 full with the slow cooker onion soup. Top each with a toasted bread slice and about 2 tbsp of cheese mixture.

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