Craig, I'm coming in late to this posting but, be sure to use red ripe Roma
tomatoes, cut of the end, squeeze out the seeds and juices. Lay them on their
side so that you would be slicing through the stem area parallel to the table.
Slice in half. Then lay both halves flat side down, and slice downward longest
length so that you halve them again, and cut fairly large dice in relation to
the onion which is a smaller dice.

The trick is to remove the placenta and seeds and then slice lengthwise.