Preheat oven to 350°. In a large skillet, heat butter over medium-high heat. Add onion; cook and stir until tender. Sprinkle chicken with paprika, salt and pepper; place in an ungreased roasting pan. Spoon onion mixture over chicken. Add water. Bake, covered, 1-1/2 hours or until chicken juices run clear., Remove chicken and keep warm. Pour drippings and loosened browned bits from roasting pan into a saucepan. Skim fat. In a small bowl, mix cornstarch and cold water until smooth. Stir into pan juices with onion. Bring to a boil; cook and stir until thickened, 1-2 minutes. Remove from heat. Stir in sour cream. Serve with chicken.