The stress is on the "Nions" - and as you will know, "O'" means "son of", so the vegetabe reference is probably just a kind of folk etymology he'll be all too familiar with
EDIT: Following Martin's insights that I somehow only saw *after* posting, I think it's very well possible that his ancestors used the apostrophe to indicate stress in order to *prevent* their name being pronounced as "onion". Which doesn't exclude the possibility that it actually *was* pronounced that way initially ... That's something you see quite a lot around here, people adapting (mostly having adapted) surnames by spelling them in a fancy way ...
He's British, living near the Lake District (he's secretive about it, as I would be, but he also makes images in his "local forest", and what you see on the horizon is telling ...).
I really like his style, and you can learn a lot (about film photography, and he also has an amazing eye).
M.