Trim the cone shaped tops from the okra, wash, drain and place in a dish. Sprinkle with salt and vinegar and set aside for half and hour. Was them again and then dry thoroughly.
Heat the olive oil in a large frying pan and add the chopped onion. Cook gently until the onion is tender. Add the okra and cook, tossing lightly, until they are slightly browned. Add the tomatoes, peeled and chopped, salt and pepper. Cover the frying pan and simmer for about 45 minutes, or until tender.