Begin by slicing off both ends, leaving some of the root end intact to help hold the onion together.
Remove and discard the dry layers of peel.
Set the onion on your work surface, cut side down, to steady it. Slice it in half, lengthwise.
Lay the halves on your work surface, cut side down.
Using your knife, make vertical slices into the onion half, being careful not to cut all the way through the end.
Now make horizontal slices. Again, not cutting all the way through the end.
Finally, make downward slices and voila, there’s your diced onion.