A different method, requires a little practise, but quick & easy once you have it. This method keeps the seeds, but loses the harder green bit where the stalk was attached.
This is the quickest manual method I have ever seen & produces evenly sized chunks - it takes considerably longer to explain than to do. It requires a razor-sharp knife, otherwise you tend to just push the tomato out of your grip & lose control of it.
Place the tomato stalk side down, for initial stability*.
Make an even number of vertical cuts, parallel to each other - so you get 3, 5 or 7 equal-thickness slices, depending on tomato size & required chunk size. Don't let go, keep the tomato complete in your hand.
Now it gets trickier, because you have to constantly keep hold...
All horizontal cuts are only half way. All vertical cuts are right through to your board.
You no longer need to count how many cuts for the rest of the task.
Make several horizontal cuts at 90° to your first ones, parallel to your chopping board - but only half way through. It's more stable if you start at the bottom & work upwards. (Yes, you are cutting towards your own hand - be careful)
Slice again a couple of times vertically, at 90° to both existing cut directions, until you reach about halfway (all these 'halfways' become clear by the end) - & off come your first few chunks; slide to one side with your knife.
Rotate the remainder onto the flat face.
Make similar horizontal & vertical cuts; freeing your second set of pieces.
Rotate the remainder again, onto the face you just cut.
Depending on the tomato size, you may have room for one last horizontal cut, then make a last set of vertical cuts.
Done.
If you got the first counted cuts just right, your 'stalk' piece is separate & ready to discard.
* I've tried this both ways up, as it always feels as though being able to see the stalk when making the first cuts would be better - it isn't, you lose stability & are likely to lose control of the tomato half way through.
Late edit
Adding some photos I took while I had my camera set up for another question…
I needed a spare hand for the shutter release, so the knife itself is always just resting in place, making it look a little odd. Click any image for full size
Stalk down, first vertical cuts. You have to straddle the tomato, as you can't let go at any time, or it will fall apart.
Still stalk down, first horizontal cuts.
Still stalk down, second vertical cuts. Cut down all the way to the board, but only cut to halfway along the tomato, so you stop just short of where the stalk is. This is our last cut now before we flip it for the first time.
Each time you complete one 'side' you flip the tomato onto the just-cut face, until the very end. Last horizontal cut.
…and you're left with the tough stalk bit, separate from the rest
I have to admit, I've done this better when I was concentrating more on the tomato & less on photographing it ;) One or two bits in the photos look like wedges not cubes. Done properly, you get very even pieces.
Notice, though, that because we've not been beating the tomato up as we did this, each piece still has its own seeds attached, they're not all in a loose pool. Of course, they will easily separate as soon as you start mixing in with a pico etc, but if you just slide them onto a plate from here, they do retain their shape better than you'd expect.