Quarter your onion
Now, make half-inch cuts TO the back but not THROUGH the back of the quarter
Here's the view from the bottom (?) of the quarter.. half-inch sections... go all the way across
now flip your knife and make one cut half way up the quarter, again TO the back but not THROUGH the back
start dicing, half-inch cuts up the onion, creating little onion cubes
voila!
here's the stump... save it for stock, toss it... whatever you want!