Remove two rings of pineapple from the tin, chop and set aside. Save the rest of the pineapple in the tin for another use. Spoon 2 tablespoons syrup from the pineapple tin and reserve.
In a small saucepan, bring some water to the boil and add the sliced green pepper. Simmer for 3 minutes until the pepper softens. Drain and set aside.
Mix the cornflour in a little cold water. In a food processor, combine the cornflour, the 2 tablespoons of reserved pineapple syrup, chutney, tomato purée, soy sauce, sugar, vinegar and 300ml (1/2 pt) water. Blend until smooth.
Melt the margarine in a saucepan, add the sliced onion and cook and stir for 3 minutes until soft. Add the mixture from the blender, bring to the boil then lower the temperature to medium and simmer for 10 minutes. Add the pineapple and cooked pepper and simmer for a further 5 minutes.