I know there are probably many ways to easily cut an onion but this is the way I use most often for small chopped up onion. First cut off the end of each side so it sits without rolling, and so you have a flat surface to start with. Then make deep cuts in both directions. Like this:
Next turn your onion on its side and carefully cut full slices from top to bottom. Like this:
And there you have it- perfect diced onion pieces.