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Heat butter in a large, heavy pot over medium-high heat; cook and stir onions until they become translucent, about 10 minutes.
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Sprinkle onions with sugar; reduce heat to medium.
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Cook, stirring constantly, until onions are soft and browned, at least 30 minutes.
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Stir in garlic and cook until fragrant, about 1 minute.
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Stir sherry into onion mixture and scrape bottom of pot to dissolve small bits of browned food from the pot.
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Transfer onions into a slow cooker and pour in beef broth.
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Season to taste with sea salt; stir in thyme and bay leaf.
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Cover cooker, set on High, and cook 4 to 6 hours.
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If desired, set on Low and cook 8 to 10 hours.
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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.
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Arrange bread slices on a baking sheet.
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Broil bread slices until toasted, 1 to 2 minutes per side.
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Fill oven-safe soup crocks 3/4 full of onion soup and float a bread slice in each bowl.
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Top with 1 slice of cheese on each soup.
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Place filled bowls onto a baking sheet and broil until cheese topping is lightly browned and bubbling, about 2 minutes.