Cut off the onion stem, but keep the root. (It’s going to hold the onion together when you chop.)
Cut the onion in half through the root. (It is sometimes easier to peel it when it’s sliced than when it’s whole.)
Peel the onion.
Take one half of the onion and lay it flat-side down on the cutting board. Slice through the onion vertically, moving across from one side to the other. Slice close to the root but not through it. The closer together your slices are, the finer dice you’ll get.
Make two slices through the onion horizontally, at a slight downward angle.
Holding the onion together with your hand, slice across the vertical cuts to create a dice as large or as fine as you need.