Mix the sherry/rice wine and soy in a bowl and add the pork and marinate for 20 minutes or more if you have the time.
if you are using tinned pineapple, open the can and drain the pineapple keeping the juice (add to the sauce) or defrost the frozen chunks.
Heat the oil in a large frying pan or wok and stir fry the pork. For 3-4 minutes until browned and not showing any pink. Add the pepper, onion and pineapple and cook for a further 3-4 minutes. Stir in the cornflower and cook for 1 minute. Mix the sauce ingredients together and add to the pan. Simmer for 3-4 minutes and serve immediately with egg fried rice or noodles.