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1
Melt the butter in a large enameled cast-iron casserole.
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2
Add the onions and a pinch of salt, cover and cook over moderate heat, stirring once or twice, until the onions soften, about 10 minutes.
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3
Uncover and cook over moderate heat, stirring frequently, until the onions are lightly browned, about 40 minutes.
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4
Stir in the sherry.
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5
Add the stock and bouquet garni and bring to a boil.
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6
Cover and simmer over low heat until the soup has a deep flavor, about 30 minutes.
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7
Discard the bouquet garni and season the soup with salt and pepper.
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8
Preheat the oven to 350.
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9
Butter the bread on both sides and place on a baking sheet.
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10
Toast the bread for 15 minutes, turning the slices halfway through, until golden and crisp but not dried out.
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11
Raise the oven temperature to 425.
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12
Bring the soup to a simmer, ladle it into 4 deep ovenproof bowls and sprinkle with half of the cheese.
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13
Place a crouton in each bowl and sprinkle on the remaining cheese.
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14
Bake the bowls of soup on a baking sheet in the middle of the oven for 10 minutes, or until the cheese is bubbling.
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15
Serve hot.
