“The leaves or the blades of the Leek be long, somewhat broad, and very many, having a keel or crest in the backside, in smell and taste like to the onion. The stalks, if the blades be not often cut, do in the second or third year grow up round, bringing forth on the top flowers made up in a round head or ball as doth the Onion.” (Gerard, John “The Herbal” 1633)
Onions and leeks look very similar when they flower. They way to tell them apart is that the leaves of the leeks are board and flat, while those of the onion are round and hollow. Here is a full length view:
Leeks were used in cookery, but beware they are very “hot” in temperature and may offset your humors as this passage warns:
It heateth the body, ingendreth naughty bloud, causeth troublesome and terrible dreams, offendeth the eyes, dulleth the sight, hurteth those that are by nature hot and choleric, and is noysome to the stomach, and breadth windiness.” (Johnson, Thomas ed. Gerard, John “The Herbal” 1633, pg. 174-175)
So go ahead enjoy your leeks, but beware of impending windiness.