Cut the tomatoes into small pieces, and finely chop the onion. The smaller the better.
Add the tuna, tomatoes and onion to a bowl, and add salt and pepper. If the tuna was canned in water, you may want to add olive oil. Also, you can add lime juice if you wish.
Mix well, and serve with crackers on the side. Done!
Notes
If the tomatoes are acidic, you may want to skip the lime juice.
Chop the onions as finely as possible, and use only a small amount, otherwise they may overpower the salad.