1. Combine beef with oyster sauce, cornstarch and 1 tsp of oil in a bowl. Set aside.
2. Heat remaining oil in a frying pan or wok over high heat. Stir fry onion for 1-2 mins. Add beef and stir fry for 2-3 mins. Add bell peppers and stir fry for 3 mins.
3. Stir in pineapple and sweet & sour sauce. Cook until heat through for 2-3 minutes.
