Finely chop all the vegetables and keep the onion separate.
In a saucepan, simmer the tinned tomatoes with some salt and pepper until it has thickened and reduced.
Cook the onion in a large pan over moderate heat until it has softened then add the zhuccini and capsicum. When they have softened and the tomatoes have reduced, add the tomatoes to the pan. Simmer it all until it thickens. I added a tablespoon of tomato paste just to intensify the tomato flavour but you don't have to.
Season to taste.