Cut the onion in half through the stem and root-ends.
Peel off the skin and possibly one layer of each half using a paring knife. Trim about 1 cm from the stem-end of each half, but during the rest of the chopping process, keep the root-ends intact; otherwise the onion will fall apart.
Lay each half, flat-side facing down, on a chopping board. Hold the onion steady with the fingertips of one hand. With the other hand, make a series of horizontal cuts from the stem towards 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 crossways to dice into evenly-sized pieces. The size of the pieces depends on how far apart you make each horizontal, vertical and crossways cut.