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1
Melt 3 tablespoons of the butter in a large enameled cast-iron casserole.
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2
Add the sliced onions, thyme sprigs and a pinch each of salt and pepper and cook over moderate heat, stirring occasionally, until the onions soften and release their liquid, about 20 minutes.
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3
Add the sugar and cook, stirring often, until the exuded liquid evaporates and the onions are caramelized, about 30 minutes longer.
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4
Transfer the onions to a medium bowl and discard the thyme springs.
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5
Melt 1 tablespoon of the butter in the casserole.
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6
Add the celery, garlic and herbes de Provence and cook over moderate heat, stirring, until fragrant, about 3 minutes.
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7
Raise the heat to high and add the sherry.
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8
Cook, stirring up the browned bits from the bottom of the casserole, until the sherry has reduced by half, about 3 minutes.
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9
Return the onions to the casserole, add the chicken stock and simmer over low heat for 30 minutes.
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10
Preheat the oven to 400.
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11
Melt the remaining 2 tablespoons of butter in a small pan.
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12
Spread the bread cubes on a rimmed baking sheet and toss with the melted butter.
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13
Sprinkle with the Parmesan and toss again.
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14
Bake the croutons for about 8 minutes, or until golden brown and crisp.
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15
Add the heavy cream to the soup and simmer over low heat for 10 minutes.
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16
Working in batches, puree the soup in a food processor or blender until smooth.
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17
Return the soup to the casserole and reheat gently.
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18
Season with salt and pepper.
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19
Ladle the soup into bowls, garnish with the cheese croutons and serve.
