Heat oven to 400ºF. Heat chicken, onions, celery, mixed vegetables and chicken broth to boiling in 5-quart Dutch oven. Stir cornstarch into cold water until dissolved; stir into chicken mixture. Heat to boiling, stirring constantly. Boil and stir 1 minute; remove from heat. Stir in parsley. Pour into ungreased rectangular pan, 13x9x2 inches.