Heat half the oil in a wok or large skillet set over high heat. Quickly cook prawns until just cooked through, about 2-3 minutes. Remove and reserve. Heat remaining oil in skillet, and then stir-fry onions and peppers 2 minutes. Add tomatoes and pineapple and cook 1 minute more. Add remaining ingredients and bring to a simmer. Add prawns, heat through, and serve.