To dice an onion, start by cutting off the top. However, leave the root end intact. This will hold the onion together while you dice.
Place the onion cut side down, slice in half, cutting from the root end down.
Peel off the skin and place the onion half flat on your cutting board. Make horizontal cuts up the onion, spacing them the same width that you want your finished dice to be. Quick Tip: remember to let your knife do the work for you by pulling the knife as you slice.
Now make vertical cuts, spaced the same width as your horizontal cuts, stopping just before you reach the root. Continue the vertical cuts all the way across the onion.
Finish your dice by cross-cutting the onion the same width as your vertical cuts.
If yield is more important than the precision of your dice, you can also dice the root portion of your onion. Flip your onion end root side up, make downward vertical cuts, and then cross-cut around the root to yield a rough dice.