Combine the pineapple juice, cornflour, sugar, vinegar, tomato puree or passata and 80 ml of water in a bowl. Set aside.
Put the pork and flour in a plastic bag and shake to coat the pork with flour. Remove the excess flour. Heat 1 tablespoon oil in a wok over high heat. Add pork and stir-fry in batches for 2-3 minutes until pork is browned. Add more oil if necessary. Remove from wok and set aside.
Heat the remaining oil in the wok then add the onion and sauté over medium heat for 3 minutes. Add the capsicum, mushrooms and pineapple then sauté for 2 minutes.
Add the pineapple juice mixture and pork to the wok; stir. Bring to the boil and cook for 2 minutes until the sauce thickens. Serve.