Chop the tomatoes, the onion and the green pepper into small diced cubes.
Mix them in a large bowl and add 5 tablespoons olive oil, 1 tablespoon of any kind of vinegar you like and a pinch of salt. Balsamic vinegar would be a great choice if you like to give the leeks salad an intense taste and it would also look great due to its dark color.
Slice the boiled leeks in half lengthwise. If you desire, you can also slice each leek in two equal halves.
Finally, place the leek slices orderly in a big dish and season with the previous mixture.