Chestnut Soup with Sourdough Sage Croutons

Put oven rack in upper third of oven and preheat oven to 375°F.

Spread chestnuts in 1 layer in a large shallow baking pan, then roast in oven until dry and slightly darkened, about 20 minutes.

While chestnuts roast, wash leeks in a bowl of cold water, agitating them, then lift out and drain well.

Heat oil and butter in a 5- to 6-quart heavy pot over moderately high heat until foam subsides, then sauté 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.

Purée soup in 3 batches in a blender until smooth (use caution when blending hot liquids), transferring to a bowl.

Pour purée 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.

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.

Ladle soup into bowls and sprinkle each serving with about 1/3 cup topping.

•Topping can be made 3 days ahead and cooled completely, then kept in an airtight container at room temperature. Reheat in 1 layer on a baking sheet in a 350°F oven until bread is crisp, 10 to 20 minutes.•Soup, without Sherry, can be made 3 days ahead and cooled completely, uncovered, then chilled, covered. Reheat and stir in Sherry before serving.