In a flameproof casserole bring the potatoes, sorrel and 10 cups of water to a boil. Season with salt and pepper. Reduce heat and simmer for 20 minutes. PurÄe the soup in a blender or food processor. Pour the soup into a soup tureen and stir in the chives. Cover and stand for 5 minutes. Stir in the cream and chervil before serving.