Drain the tin of pineapple, retaining the juice. Roughly chop the pineapple and put in a saucepan with the juice, garlic, tomato purée and vinegar. Bring to the boil and simmer for 5 minutes, then purée in a blender.
Heat the oil in a wok until hot and stir-fry the chicken and sliced pepper for 5-6 minutes. Add the babycorn, ginger and chilli, then cook for 2 minutes more. Stir the sauce into the pan and heat through.
Meanwhile, cook the rice according to the instructions on the packet. Serve with the chicken and garnish with the sliced spring onions.