Leeks, a member of the onion family, bring a subtle, sweet depth to many dishes. Their long, white stems and fluffy green tops are both edible, although the white and light green portions are most commonly used in cooking. Cleaned and sliced leeks can be sautéed to form a delicate base for soups like the classic French potato leek soup, or included in quiches and pies for a mellow flavor. When cooking with leeks, it's important to rinse them thoroughly to remove any dirt trapped within their layered leaves.