Slice tomatoes in half, and use a spoon to scoop out the seeds. Dice into very small pieces. Dice the onion into very small pieces. Slice the jalepeño in half, and use a knife to cut out the membrane and seeds. Dice into very small pieces.
Combine all ingredients, starting with 1/2 of the onion and stir together. Taste and add salt, cilantro, onion, and/or lime juice to taste. Allow to rest in the fridge for an hour before serving.