from How to Cook Italian, by Giuliano Hazan

1.  Cut off the top of the onion, but not the root end with a chef’s knife, then cut the onion in half lengthwise and remove the skin.

2.  Place the onion cut side down and use the tip of your knife to make lengthwise cuts close together, from the outside of the onion to the center, making lots of narrow wedges.

3.  Thinly slice the onion crosswise.  You now have finely chopped onion.