One of the lovely lessons I learned in my knife skills class (besides how not to cut off your finger which I promptly forgot after graduating), was how to quickly dice an onion. And, now I share:
Trim off each end of the onion. Leave the back end so that there is still a little nub.
Pull off the outer layer of skin.
Cut the onion in half lengthwise.
Lay the cut end on the cutting board and make two to four horizontal slices. This is where it is imperative that you have a sharp knife. If you don’t, you’re just going to slice your hand.
Turn the onion toward you so that the little nub is away from you. Make vertical cuts across the entire onion surface.
Now, chop the onion from front to back.
The slices that you have made along with the layers of the onion, will form a dice. The size of the dice depends on how wide you made those vertical lines.