Want to get a perfectly diced onion? All it takes is practice and a sharp knife.
Start by peeling your onion, but make sure you leave the root intact. Cut in half lengthwise. Place the onion halves flat-side-down on a board. Make cuts lengthwise, at regular intervals, across the onion, leaving the root end attached, ie, don’t cut all the way along the onion. Turn the onion and cut across the other cuts, so you have nice small dice. Repeat for the other half.
Chefs often make a horizontal cut through the middle of the onion half towards the root before doing the final chop. This isn’t strictly necessary, but it does make for nice small dice.