Heat the olive oil in a large, deep frying pan. Add the onion and fry over a medium heat for 5-10min until softened. Add the pepper and fry for a further 2-3min.
Stir in the tomatoes and vinegar, bring to the boil and cook for 1-2min, then add the fish and poach gently for 10min, adding a splash of water if the sauce becomes too thick. Stir through the cilantro, season with salt and pepper and serve.