A member of the onion family, leeks resemble oversize green onions, with wide green leaves, a fat white stalk and roots at the bulb end. Yet, the flavor of leeks is very subtle. Buy leeks with crisp, brightly colored leaves and an unblemished white stalk. Leeks that are larger than 1-1/2 inches in diameter will be less tender. Refrigerate leeks in a plastic bag for up to 5 days. Before using, cut off the roots. Trim the tough leaf ends. Slit the leek from end to end and wash thoroughly under cold water to remove dirt trapped between the leaf layers. Chop or slice the white portion to use in a variety of recipes.