Green onions (aka scallions) do in fact regrow, but like others have pointed out, if you are havesting in-ground green onions for immediate use, you should leave them in the ground and just cut off the portion above ground so they can grow back again. I have not yet tried with regular onions, but if I ever succeed in GROWING those in the first place (epic fails so far), I am going to try that also.
Green onions (aka scallions) do in fact regrow, but like others have pointed out, if you are havesting in-ground green onions for immediate use, you should leave them in the ground and just cut off the portion above ground so they can grow back again. I have not yet tried with regular onions, but if I ever succeed in GROWING those in the first place (epic fails so far), I am going to try that also.