Heat butter in a large, heavy pot over medium-high heat; cook and stir onions until they become translucent, about 10 minutes.

Sprinkle onions with sugar; reduce heat to medium.

Cook, stirring constantly, until onions are soft and browned, at least 30 minutes.

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

Stir sherry into onion mixture and scrape bottom of pot to dissolve small bits of browned food from the pot.

Transfer onions into a slow cooker and pour in beef broth.

Season to taste with sea salt; stir in thyme and bay leaf.

Cover cooker, set on High, and cook 4 to 6 hours.

If desired, set on Low and cook 8 to 10 hours.

About 10 minutes before serving if putting bread and cheese on, set oven rack about 8 inches from the heat source and preheat the oven's broiler.

Arrange bread slices on a baking sheet.

Broil bread slices until toasted, 1 to 2 minutes per side.

Fill oven-safe soup crocks 3/4 full of onion soup and float a bread slice in each bowl.

Top with 1 slice of cheese on each soup.

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