How do I cut a blooming onion?
Slice off 1/2" of the top of the onion (opposite the root)
Place onion root side up, and make slices all the way through, staying at least 1/2" away from the root
Flip it over and spread apart the leaves


This is best described with images.

Alternatively, if you're willing to drop a few hundred dollars, you can buy a bloomin' onion cutter. This is what restaurants use. There are cheaper alternatives, but I can't vouch for their quality or ease of use.