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1
In a large Dutch oven or other large pot, melt 3 tablespoons butter over medium heat. Add the oil and onions; cook onions are until softened, stirring occasionally, about 15 minutes. Add the salt, pepper, and sugar; continue to cook, stirring occasionally, until onions are deep golden brown and caramelized, reducing heat slightly if onions seem to be browning too quickly, 35 to 45 minutes more.
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2
Add wine and raise heat to high. Cook until almost all liquid has evaporated, 8 to 10 minutes.
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3
Tie thyme and bay leaves into a bundle with twine. Add broth and herb bundle to pot with onions. Bring to a boil, reduce to a simmer, and cook, uncovered, until broth is thickened and flavorful, 20 to 30 minutes. Remove from heat and whisk in remaining 2 tablespoons butter. Taste and adjust seasoning.
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4
Heat the broiler. Cut two 1/2-inch baguette slices for every serving of soup. Place baguette slices on a rimmed baking sheet and toast in oven until crisp and dry but not browned, about 1 minute per side. Rub one side of each toast with the garlic clove and set aside.
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5
Place ramekins or ovensafe bowls on a rimmed baking sheet, add 1/2 teaspoon of sherry to the bottom of each, and ladle soup on top. Top each serving of soup with two garlic-rubbed toasts. Divide cheese among the servings, covering the bread and some of the soup. Carefully transfer baking sheet to oven and broil until cheese is melted and bubbling, 4 to 8 minutes. (Alternatively, if using regular soup bowls: Top each garlic-rubbed toast with some cheese and return to broiler to melt, about 2 minutes more. Divide sherry and soup among bowls, sprinkle with remaining cheese, and top each serving with two cheese toasts.)
