Place first 3 ingredients in a mortar and pestle or miniature food processor; mash until a smooth paste forms. Spoon mixture into a medium bowl; add sour cream, and stir well. Stir in chicken, chiles, and onion. Cover and chill 2 hours. Serve with fresh raw pepper slices or other raw vegetables, Melba toast, or unsalted crackers; or use as a sandwich spread.