The taste and appearance of this dish are those of a stew. The tuna is cut into cubes and it is served with some of the things that are kindest to it, slowly cooked onion combined with tomato and chili pepper. We know, however, that we cannot subject tuna to the unhurried cooking stews require because its meat stays tender until it just begins to feel the heat, and then starts to turn irreparably tough and dry. The solution is to cook the onion and tomato for as much time as necessary, but