How to select for best flavor:
Choose well-shaped, medium-sized leeks with fresh, green tops. The root end should have several inches of blanched or white skin. Avoid leeks with obvious signs of age or mishandling, such as ragged or wilted greens or split bulbs.
Peak of the season:
Leeks are available year-round, but the peak months are September through November. There is another small peak in May. Most West Coast leeks come from California. On the East Coast, they come from New Jersey.
An average bunch of leeks contain only about 50 calories. Leeks are richer in nutrients than other members of the onion family, especially in vitamin C, calcium and potassium. They are also an excellent source of vitamin E, which is hard to come by in vegetables.
Leeks have far fewer calories and carbohydrates than onion, as well as much higher vitamin content. For this reason they are a very good substitute for onions, although the taste is milder and more subtle. The leek has a very rich history. The Egyptians regarded it as an embodiment of the godhead. The Roman army distributed it all through Europe, where it became so popular that it actually became the national plant of Wales.