Leeks are a vegetable similar in appearance to a large green onion or scallion. Like the onion and garlic, a leek is part of the asparagales alliaceae family of herbaceous perennial flowering plants. It's milder in taste than the onion, with only the white base and light green stalk typically used for eating while the darker green portion is tossed away. Leeks are served fried, boiled or raw, and in combinations with other ingredients.
Boiled leeks are often found in soups, such as leek and potato soup or Scottish cock-a-leekie made with chicken, prunes and whipping cream. Combined with buttermilk, heavy cream and broth, leek and potato soup is a hearty dish, and Vichyssoise is the famous cold version.
Though leeks feature prominently in several soups, many cooks find that simply boiling them, and then adding a bit of salt and pepper to taste, makes an excellent side dish.
Frying leeks makes them crunchy and still maintains their delicate flavor. Cutting leeks into julienne (long thin strips) before frying is a convenient way to make a matchstick treat. Deep-fried in batches until golden brown, these matchstick leeks work well atop a steak, as a garnish on other dishes or as a tasty side dish when tossed with a little sea salt.
Leeks sautéed with butter and served alongside steamed or broiled fish make a nice variation on deep frying. Martha Stewart offers a recipe for Sautéed Green Apples and Leeks recipe that contrasts the vegetable and fruit flavors (see References).
Leeks are often added to salads. Tossed with a vinaigrette, vegetable or olive oil, leeks go well with fresh tomato, lemon juice, parsley, cucumber, garlic and olives. Martha Stewart's Leeks Vinaigrette is a tangy treat; the recipe calls for grainy Dijon mustard.
In addition, many recipes call for leeks to be added to casseroles and quiches, tossed with pasta and shellfish, or used in tarts, gratins, puddings and sauces.
Look for leeks that are dark green and very firm; ideally the roots should be crisp. Leeks can be gritty and difficult to clean because dirt and sand are often trapped in them.
Pay careful attention when cooking leeks. They are easy to overcook and turn slimy.
Though you can substitute leeks for onions, due to their milder flavor, be cautious when doing the opposite: Using onions instead of leeks can result in a strong and unwanted taste.