Heat oil with salt in wok. Add cabbage, onion and apple. Stir fry for 2 minutes. Add water and sugar. Cover and steam for about 10 minutes or until cabbage in still slightly crisp. Uncover and stir occassionally. Add corned beef. Stir fry for 1 minute. Cover and steam about 1 to 2 minutes or until corned beef in heated. Add soy sauce and dissolved cornstarch. Stir until slightly thickened.