Fresh chestnuts are a tough nut to crack, literally, so be ready to work for this oh-so-worth-it soup. Before roasting, the chestnuts need to soak in warm water for 25 minutes; after roasting, you’ll need to peel the nuts by hand while still warm, which takes time. That said, it’s a celebratory starter that feeds a crowd, and you will be very glad that you put in the effort to make it. For an ultra-silky texture, use a regular blender rather than a hand blender to purée the rich soup.
Position a rack in the center of the oven and heat the oven to 400°F.
In a medium bowl, cover the chestnuts with warm water and soak for 25 minutes. Drain and dry with paper towels. Score an X into the flat side of each chestnut, cutting all the way through the shell. Put them flat sides up on a rimmed baking sheet and roast until the Xs curl back into a crown shape and they are tender when squeezed, about 30 minutes. Peel while still warm, removing both the shell and the inner skin. If a shell resists peeling, spoon out the nutmeat instead. Roughly chop the chestnuts and set aside in a medium bowl.
In a 6-quart pot, melt the butter with 2 Tbs. of oil over medium-low heat. Add the leeks and cook, stirring occasionally, until softened, 7 to 8 minutes. Add the chestnuts and 1 Tbs. of the thyme; cook until fragrant, about 1 minute. Add the broth and 1/2 tsp. salt; bring to a boil over medium-high heat. Reduce the heat to low and simmer until the chestnuts are extremely tender, about 30 minutes, skimming off any foam that rises to the surface. Let cool briefly. Working in batches, purée the soup, either in a regular blender or with a hand blender. Strain the soup through a fine sieve into a clean 4-quart pot. Set the soup back over medium-low heat and gently reheat. Season to taste with salt and pepper.
Meanwhile, in an 8-inch nonstick skillet, heat the remaining 1 tsp. oil over medium heat. Add the prosciutto and cook, stirring frequently, until crisp, 2 to 3 minutes. Add the remaining 1/2 Tbs. thyme and cook 30 seconds more. Drain on a plate lined with paper towels.
Ladle the soup into bowls and garnish with some of the prosciutto and thyme.
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Photo: Scott Phillips