Leeks are a member of the Allium family, which is essentially the onion family, and can really be used in any way that you would use an onion, which is lots of ways. Their flavor is slightly milder than that of a typical onion. They look like oversized scallions or green onions, long and cylindrical, and they should be firm, with nice taut layers.
They are available in the fall and the spring, with the spring leeks being smaller and more mildly flavored. The dark green tops are very fibrous and tough, and can be used to flavor stocks, but it’s the light green and white parts that are best for eating. Leeks can be eaten raw or cooked, and featured as a vegetable in their own right (which is more common in European cooking) or as a supporting aromatic.
Leeks push their way out of the ground, and so quite a bit of dirt can get trapped between the layers. Wash them extremely well under cold water just before using, and if you are slicing or chopping them for a recipe, do that before rinsing them, to make it easier to remove the dirt.
Leeks will last in the refrigerator for about two weeks.
Some great recipes that include leeks: