This is the quickest way that I have found to dice an onion. It also causes the least amount of tears.
I call the top of the onion the half that the stem grew up from. The bottom is where the roots grow out.
1. Peel the onion.
2. Cut the onion straight through the top and bottom. You should have half of the root on each side.
3. Place the onion in the freezer. If you're only using half of the onion for your recipe, store half in the fridge for a later use.
4. After a few minutes, take half of the onion out of the freezer.
5. Place it flat side down on a cutting board.
6. With a sharp knife, cut the top tip off of the onion.
7. Hold your knife parallel to the cutting board.
8. Slice horizontally through the onion from the top towards the roots, but not through them.
9. Make more cuts for smaller pieces and less for larger ones. Two should be fine.
10. Turn your knife so that it is vertical to the board again.
11. Place the tip of the knife, next to the root on the far end of the onion.
12. Slide your knife down to make a cut.
13. Moving down the onion, make a few more cuts like that (close to the root, but not through it.)
14. Turn the onion so that your knife is parallel to the cut you made when you sliced off the top of the onion.
15. Now, cut the onion going in that same direction a few times. The onion should fall down off your knife in nice little squares.
16. When you're done , take the second onion half out of the freezer and repeat.
Hint: Don't leave the onion in the freezer too long. Sounds crazy, but it will freeze.
More importantly, keep your fingers tucked under, away from the blade.