Cut the onion in half through the stem and root ends.
Peel off the skin and possibly one layer of each half with a paring knife. Trim about ½ inch from the stem end of each half, but during the rest of the chopping, keep the root end on each half intact; otherwise the onion will fall apart.
Lay each half, flat side down, on a cutting board. Hold the onion steady with the fingertips of one hand. With the other hand, make a series of horizontal cuts from the stem toward the root end but not through it.
Then make a series of vertical cuts, with the tip of the knife cutting almost but not quite through the root end.
Then cut each half crosswise to create an evenly sized dice. The size of the dice depends on how far apart you make each horizontal, vertical, and crosswise cut.