Dicing an onion is a multi-step process as follows:

• Place the onion on a cutting board. Remove the stem end and trim the root, but leave it otherwise intact.
• Cut the onion in half through the stem and root. Remove the outer layer of onion skin and place the onion cut side down.
• Cut parallel slices of your desired thickness vertically from the root toward the stem end without completely cutting through the root end.
• Make a single horizontal cut on a small onion (or two cuts on a large onion), again without cutting through the root end.
• Turn the onion and cut slices perpendicular to the other slices to produce diced onion. Make the thickness of these slices similar to the thickness of the vertical slices.