Core the tomato and cut in half crosswise with a serrated knife to expose the seed chamber. Gently squeeze out the seeds, using a finger or a small table knife to help empty the chambers. Lay the seeded tomato halves, cut side down, on a cutting board. Holding the serrated knife parallel to the cutting board, cut the tomato halves horizontally into slices that are as thick as you want your dice to be. Next, cut each stack of tomato slices into strips as wide as you want your dice, and then slice these strips crosswise into dice.
Pros: You use the whole tomato; good for beefsteak tomatoes, which have a lot of delicious inner flesh.
Cons: Takes a little more time; the stacks can be a little awkward to slice.
Use a small table knife or a fingertip to flick out the seeds.
Cut the tomato into strips and then slice crosswise into dice, as you'd cut an onion