This is close to what I do. I cut the root end and the opposing end off, using the connecting bits to peel the onion. Then I stand it on the cut end and make vertical cuts down to about ½cm above the end of the onion, leaving everything connected. Rotate 90° and repeat until you've got a Bloomin' Onion cut. Then turn the onion 90° on its side, and make vertical cuts again until you get to the part you didn't dice. You can save this part for later quite easily, if you didn't need a whole onion; otherwise place it on the cutting board, cut-face down, and dice in a grid pattern.

It doesn't give you perfectly uniform sizes, and it's not the fastest, but it's a good midpoint between uniform and speedy.