Yeah, for people the leaves so maybe stems ok, but I definitely wouldn't feed any part of potatoes or onions myself, potatoes are too starchy for one thing and onions are toxic, but it may take time for the levels to build in my opinion. I've seen people feed them too and some sites say no toxic, others say ok.I think it's the leaves of rhubarb that are toxic for people, I've seen it suggested that the stalks are ok for tortoises since they are ok for people. Potatoes, it's also the leaves; I've seen the actually tubers listed on edible lists going back a while. I don't know of any evidence of onion being actually toxic; and have known multiple people that have fed onion with no apparent ill effects.
That said, if anyone is unsure, it's always better to play it safe.