1) To make the sauce, drain pineapple juice from the pieces and reserve. 2) Mix together tomato ketchup, soy sauce, vinegar, sugar, cornflour and seasoning. Stir in to the pineapple juice. 3) Heat half the oil in a large frying pan or wok, add onion and stir fry for 2-3 minutes. 4) Add the carrots and stir fry for a further 2 minutes. Set aside. 5) Heat remaining oil, add prawns and fry for 1 minute. Pour in the sauce and bring to the boil, stirring until thickened. 6) Add chestnuts and pineapple and cook for 2 minutes. 7) Stir in onion and carrot and cook for a further minute. 8) Serve with boiled rice or noodles.