1.Heat half oil in a wok or large frying pan on high.
2.Dust chicken in cornflour, shaking off excess. Stir-fry, in batches, 3-4 minutes each batch, until well browned. Remove from pan and set aside.
3.Heat remaining oil in same wok. Stir-fry onion and carrot 2-3 minutes, until onion is just tender. Add reserved syrup, vinegar, sugar and soy; bring to the boil, stirring.
4.Return chicken to wok with pineapple pieces and capsicum. Stir-fry a further 2-3 minutes, until bubbling and thickened. Toss buk choy through.
5.Serve chicken with rice, or noodles if preferred.