Place potatoes in a 5-qt. slow cooker. Add chicken, onions and garlic; sprinkle with seasonings. Pour 1/2 cup water over top. Drizzle with oil. Cook, covered, on low until chicken and vegetables are tender, 5-6 hours., In a bowl, mix cornstarch and remaining water until smooth. Remove chicken and vegetables from slow cooker; keep warm. Strain cooking juices into a small saucepan; skim fat. Bring juices to a boil. Stir cornstarch mixture; stir into juices. Bring to a boil; cook and stir until thickened, 1-2 minutes. Serve with chicken and vegetables.