Resembling a giant green onion, with its bright white stalk and its long, overlapping green leaves, the leek is the mildest member of the onion family. Native to the Mediterranean region and essential to classic French cuisine, leeks bring a hint of both garlic and onion to the dishes they flavor. They can serve as a component of mirepoix, a classic French seasoning mixture; meld with other ingredients such as potatoes in standard preparations like vichyssoise; or star in a gratin or quiche. Raw leeks can also be sliced and added to salads.
Choose smaller leeks with dark green leaves that are crisp, firm and free of blemishes. Check to make sure that the roots are light in color and still pliable. Avoid darkened, dried roots and wrinkled, wilted leaves.
Keep leeks in a plastic bag in the refrigerator for up to 5 days.
Because leeks grow partly underground, grit is often lodged between the layers of their leaves. Wash them thoroughly before cooking as follows: First, trim off the roots and the tough, dark green tops of the leaves. If the outer layer is wilted or discolored, peel it away and discard. Quarter or halve the stalk lengthwise. If using the leek whole, leave the root end intact.
Rinse the leek well under cold running water, separating the layers and rubbing the leaves to remove any silt between them. If a recipe calls for sliced leeks, slice the white and lighter green parts crosswise.
Adapted from Williams-Sonoma Kitchen Companion: The A to Z Guide to Everyday Cooking, Equipment and Ingredients (Time-Life Books, 2000)