Leeks are members of the Amaryllidaceae family, along with other alliums such as garlic and onions, and are used in many of the same types of dishes. These vegetables look like large, fat, green onions or scallions, and are primarily available in the spring, though smaller “summer leeks” are also sometimes available in farmer's markets. Cooks can use this vegetable in a variety of soups, sauces, and baked leek dishes, as well as fried or raw in salads and as a garnish on other foods. Leeks have a relatively mild flavor and cannot be substituted directly for onions in recipes, but pair well with mild meats like chicken and veal, as well as cheese and eggs, other vegetables, and potatoes or rice.
The leek is most closely related to the large, mild elephant garlic and kurrat, or Egyptian leek, but it also shares a genus with common garlic and onions. Like these vegetables, it has a slightly spicy flavor with sulfurous elements, and contains a relatively high percentage of vitamin C. Unlike ordinary garlic and onions, leeks and their close relatives are quite mild, and lack a strong “bite.” Like other alliums, they are used to add depth of flavor to both meats and vegetarian savory dishes. Cooks can buy overwintering leeks, which resemble spring or green onions, in supermarkets during early spring, or smaller, thinner summer leeks in farmer's markets during the warm season.
Soups are among the most popular leek dishes, including potato leek soup, Scottish cock-a-leekie, and vichyssoise. They can also be used to make sauces, such as creamy leek pasta, or fish sauce, made with cream and strong cheeses. Baked leek dishes also frequently include a dairy element, such as leek gratin, which uses heavy cream and cheese, or leeks in quiche, which pairs these delicate vegetables with cheese and eggs.
Slivered leeks make an excellent garnish or salad ingredient, whether served raw or fried in butter or oil. Fried leeks may be cooked slowly to caramelize the natural sugars in the vegetable, or quickly, as “frizzled leeks,” to produce a crispy texture. Both the cooked leek and the cooking oil can then be added to food to improve its flavor. When used in salads, these vegetables work both as garnish and as a primary ingredient, especially when accompanied by a flavorful dressing.
It's possible to substitute leeks for other alliums in cooking, but most recipes won't allow a direct one-for-one substitution due to the leek's mild taste. Substitute leeks directly only in recipes that call specifically for mild, sweet onions, shallots, or elephant garlic. Use a much greater quantity of leeks in recipes that call for ordinary onions, or reduce the intensity of other flavoring agents. Leek dishes pair well with veal, chicken, and oily fish, as well as eggs, cheese, and cream, or starchy elements like potatoes and rice.