My favourite knife to use for this is a victorionox serrated edge knife, but everyone has their favourite knife. Use what you feel comfortable with!
Cut off the root, stay as close to the root as possible as we want this end to hold the onion together later!
You need to know which is the root end. It's on the left in this pic.
Place the onion flat with the root end to the left. If your left handed you need it on the right
Slice the onion, don't cut all the way through the root end
See how far my knife goes
So you see you cut as far as you can without slicing completely
Make one slice horizontally
Again only so far! If it's a big onion you may need to do 2-3 of this slice
Lay that piece flat to make it easier to cut
And there you go! Perfectly diced onion!
This may feel a bit awkward at first but it's so worth it as once you've got it you'll dice an onion in no time at all! Let me know how you get on! Any questions?