Heat butter in a large saucepan over moderate heat. Add onion and saute until soft, about 10 minutes. Add garlic and saute another 2 minutes. Add sorrel and celery and cook for about 5 minutes, or until sorrel is wilted. Add stock or water, parsley, salt, pepper and nutmeg. Simmer for about 20 minutes.
Remove saucepan from heat and puree soup in batches in a blender or a food processor fitted with the metal blade. Adjust seasonings and transfer soup to a container. Cover and refrigerate until chilled through.
To serve, ladle chilled soup into bowls and garnish with chopped parsley and a dollop of sour cream or plain yogurt.
c.1996, M.S. Milliken & S. Feniger, all rights reserved