I'm much into the "walking onion" aka the "tree" onion or "egyptian" onion, officially described in most texts as "allium x proliferum". It's an awesome crop, but it doesn't seem much research has been done on it, it's origins are very obscure, and how exactly it's related to other alliums hard to determine.
However, I just found this site:
which indicates that the walking onions are the result of a cross between the "cultivated" onion (allium cepa, which would include common bulb onions, shallots, and "potato" onions, I believe) and allium fistulosum, aka the "Welsh" onion.
So now I'm pretty keen to get hold of that latter. I don't know whether the Welsh onion is sterile like the walking, but if anyone here has some and would swap me seed or top-sets for top-sets of my walking onion cultivar (Catawissa), that'd be great.