Start by trimming off the top stem with a knife.
Then cut the onion in half.
Now peel off all the skin.
Make vertical cuts 90% of the way through the onion. The last 10% should still be attached to the other onion stem.
Next spin the onion and press it on the table with your hand on top of the onion. Make two horizontal cuts through the onion.
Finally, chop the face of the cross section with your knife to get a nice fine dice.