So right now we have the Red Onion Helm -> http://xivdb.com/item/2820/red+onion+helm

But the only other 'onion' things we have are as follows
http://xivdb.com/item/2962/onion+doublet
http://xivdb.com/item/3279/onion+gaskins
http://xivdb.com/item/3743/onion+pattens

These are 1.0 items, which I believe you started off in, as well as a Blue Onion Helm, which was only carried over from 1.0 if you had it already

Now, these items don't go all that well together, seeing how Red Helm, Blue Armour....unless you're a Legacy Player

I'm suggestion that the actual gear from Onion Knight be added to either the gold saucer, or to the random retainer missions...I know for sure I'd be working for them, seeing how Onion Knight is one of my favorite classes in the whole FF series, as well as my personal fave character from Dissidia.

Maybe even have this stuff set up for DoW/DoM classes, and adding a mage type wooden helm for glamour too, so that way all combat classes could glamour as this, since Onion Knight was able to do just about everything in the original game the class Debuted in.

P.S. - Yes I know we have the Onion Set from Crystal Tower for Dragoon, but that doesn't count.