Heat the oil in a wok or large frying pan over a high heat. Add the pork and onion and stir-fry for about 4 minutes until the pork is cooked through.
Meanwhile, drain the pineapple, reserving the juice from the can. Whisk the pineapple juice with all the sauce ingredients and set aside.
Add the pepper and pineapple to the wok and stir-fry for 2–3 minutes, then pour in the sauce. Cook, stirring continuously, until the sauce thickens. Serve immediately with noodles.