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, mix together the soy sauce, tomato ketchup, reserved pineapple juice and vinegar in a small bowl.
Heat the oil in a wok. 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 vegetables are piping hot. Serve the pork with the rice, garnished with the remaining spring onions.