Bring a large pan of water to the boil. Add the rice and cook for 20 minutes. Drain, then stir through half the sliced spring onions.
Meanwhile, in a small bowl, mix together the soy sauce, tomato ketchup, the reserved pineapple juice and the vinegar.
Heat the oil in a wok or large frying pan, add the pork and stir-fry for 5 minutes. Add the peppers and stir-fry for 3-4 minutes, then add the drained pineapple chunks, most of the remaining spring onions, the pak choi and the sauce. Stir-fry for 2 minutes until the pork is cooked through and the veg is piping hot.
Serve the pork with the rice, garnished with the remaining spring onions.