an Asian plant of the lily family widely cultivated for its pungent smelling, strong tasting edible bulb that is used as a vegetable has a brown papery skin: Allium cepa
the bulb of this plant, used as a vegetable
know one's onions
Brit, informal to be well informed about a subject
[probably short for onion rings, rhyming slang for ‘things’. The phrase is one of a number of food-related expressions, including know one's oats, oil, apples, and eggs, as well as the more general word stuff.][Middle English via French from Latin union-, unio, perhaps from unus one]