Cut the tomatoes into pieces, stew until soft and press through a sieve. Liquidise the capsicum, onions and spices in a blender.
Cook the tomato pulp and vegetable mixture in a saucepan over a moderate heat. Bring to the boil and simmer for at least an hour. Mix the cornflour with water and stir into the tomato mixture. Bring to the boil again.
Pour the hot mixture into a sterile jar and store in the fridge.