The leek (Allium ampeloprasum) is the gentlest of the cultivated onions — a tall, cylindrical stalk of tightly furled blue-green leaves blanching to a long white shaft at the base. Sweet, mild and faintly oniony, it melts into a silky, savoury sweetness when cooked, the backbone of soups, braises and the classic leek-and-potato pairing.