Leeks are a member of the onion family with a mild, sweet flavor. They are a good source of vitamin K and vitamin C, as well as other beneficial plant compounds. Leeks have a long white stem and green leaves, and are typically used as a flavoring vegetable in soups, stews, and casseroles. When selecting leeks, look for ones that have a firm, unblemished white stalk and bright green leaves. Store leeks in the refrigerator in a plastic bag for up to a week. To prepare leeks, trim off the root end and the dark green leaves, and slice the remaining white and light green portion into thin rounds or strips. Leeks can be cooked by sautéing, roasting, or boiling until tender.