Chestnut and celery soup

2 cups prepared unsweetened chestnut puree or 1 1/2 lb fresh chestnuts
1/2 cup unsalted butter
2 white onions, chopped
3 celery stalks, coarsely chopped
1 large russet potato, peeled and coarsely chopped
6 cups chicken stock
2 cups heavy cream
1 tablespoon salt
1 teaspoon ground white pepper

1. If you are using prepared chestnut purйe, set aside. If you are using fresh chestnuts, preheat an oven to 400 degrees F. Using a sharp knife, cut an X on the flat side of each chestnut. Spread the chestnuts in a shallow pan and roast until the nuts feel tender when pressed and the shells have curled where cut, 25-30 minutes. Remove from the oven and, using a small, sharp knife, remove the shells and the furry skin directly under them. (The nuts peel easiest when still warm.) Set aside.
2. In a large saucepan over medium-high heat, melt the butter. Add the onions and celery and sautй until golden brown, about 5 minutes.
3. Add the chestnut purйe or roasted chestnuts, potato, chicken stock and cream, stir well and bring to a boil. Add the salt and white pepper, reduce the heat to medium-low and simmer, uncovered, until the soup thickens slightly, about 1 hour.
4. Working in batches, transfer the soup to a blender and blend on high speed until smooth and creamy, about 1 minute.
5. Return the soup to the saucepan and bring to a simmer over medium heat. Taste and adjust the seasoning. Ladle into warmed bowls and serve immediately.