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1
Put oven rack in upper third of oven and preheat oven to 375u00b0F.
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2
Spread chestnuts in 1 layer in a large shallow baking pan, then roast in oven until dry and slightly darkened, about 20 minutes.
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3
While chestnuts roast, wash leeks in a bowl of cold water, agitating them, then lift out and drain well.
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4
Heat oil and butter in a 5- to 6-quart heavy pot over moderately high heat until foam subsides, then saute leeks, onion, celery, carrots, and bay leaves, stirring occasionally, until vegetables are pale golden, 8 to 10 minutes. Add chestnuts and tamari and cook, stirring, until most of liquid is evaporated, 1 to 2 minutes. Add water (2 1/2 quarts) and salt and simmer, uncovered, stirring occasionally, until vegetables are tender, about 30 minutes. Remove from heat and stir in parsley and sage. Discard bay leaves.
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5
Puree soup in 3 batches in a blender until smooth (use caution when blending hot liquids), transferring to a bowl. Pour puree through a medium-mesh sieve into cleaned pot, pressing hard on and then discarding solids. Bring soup to a simmer and stir in Sherry, pepper, and salt to taste, then remove from heat.
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6
Heat oil and butter in a 12-inch heavy skillet over moderately high heat until foam subsides, then cook bread cubes with sage, turning occasionally, until croutons are golden, 6 to 10 minutes. Add chestnuts and cook, stirring, 2 minutes. Transfer to a large bowl and toss with sea salt.
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7
Ladle soup into bowls and sprinkle each serving with about 1/3 cup topping.
