Onions (Allium cepa) belong to the same plant family as garlic, shallots and leeks. While it is considered a vegetable in its own right, it is also widely used as a flavoring ingredient in many dishes as well as in many packaged foods. Flavor and pungency vary among the varieties, ranging from the mild taste of a scallion to the strong bite of a Spanish onion.
The terms "green onions" and "scallions" are generally used interchangeably despite the fact that there are differences between them. A green onion is the small immature stage of a full-sized storage onion and can be grown from any variety. Its bulb is round, mildly flavored and is a perfect tiny replica of a large mature onion. If left in the ground, it eventually grows to full size. A scallion, also called a bunching onion, does not develop a round bulb no matter how long it stays in the soil, and its flavor is more concentrated and pungent. Both are used the same way by incorporating either the white part only of the onion or the entire plant, including the upper green stalk.
Standard, or dry, onion varieties are grown with an eye toward a long shelf life. Curing, or allowing them to dry in a warm, airy place right after harvesting, is part of the process that allows them to develop the thick papery outer skin that protects them in storage. Onions grown for storage come in yellow, red and white varieties, and if not harvested as scallions are left in the ground until their tops yellow and fall over, indicating that growth has stopped. Onions whose tops produce flowers have gone beyond the ideal time for harvesting and should be used right away, as this makes them unsuitable for storage.
According to the National Onion Association, storage, or bulb, onions develop their sharp, pungent flavors over time, and the season has a lot to do with how an onion tastes. Yellow, red and white varieties represent the bulk of onions grown in the United States and are classified according to flavor and storage qualities. Yellow and red onions are available in sweet or fresh form and are identifiable by their thin skins and mild to slightly pungent taste. Cured yellow or red onions have thick papery outer skins, a much stronger flavor and store well. White onions are offered as fresh with a moderately strong flavor or as storage onions with a pronounced pungent flavor.
Yellow onions are the best choice for sautéing, as they develop a rich, nutty flavor during caramelizing. They can also be used raw in sandwiches and salads, cooked in soups, stews and roast dishes or grilled. Red onions can be eaten raw, roasted, used as yellow onions in many dishes and grilled. White onions lend themselves to grilling or sautéing, or eaten raw. Green onions, with or without their green tops, are an excellent choice for salads, stir-fries, in dips and spreads, as a garnish or in any recipe that calls for a milder onion flavor.