Every single website that suggests onion juice is good for hair regrowth doesn't have credible sources. I went through a lot and all of them are full of mumbo jumbo and charlatans.
However, this onion madness seems to have some.. I repeat some... scientific basis and maybe that's where it all started.
There is an actual research paper on the matter:
The problem though is that it was a very small study. Only 38 patients. They divided them in 2 groups and gave onion juice to one and plain tap water to the other. They noticed hair regrowth on the first group... but also on the second group!!!
Does that mean onion juice is good for hair regrowth? - Maybe, but the study is too small to prove anything.
Does that mean water is good for hair regrowth? - uh... I don't know, wouldn't there be a lot less bald people walking around?