[A very good question that many people have asked themselves! But so far no one has found the answer.

Hijikata blinks at Noah's bafflement. He only saw a small (rather shoddy and dangerous) part of his (or rather, the other timeline's Noah's) world in Queen Mab's Cavern, but they don't even have onions there?! He's going to have to ask about that.]

It's an onion. They're a type of vegetable. Do you have any of those back home? They grow in the ground and usually have green leaves poking out.