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The onion (Allium cepa) is a member of the family Alliaceae, which also includes garlic (A. sativum), leeks (A. porrum), shallots (A. ascalonicum), and chives (A. schoenoprasum). Onions were formerly classified in the family Amaryllidaceae, which includes amaryllis, daffodils, and other bulb plants; they are now considered to be between the amaryllids and the lily family (Liliaceae).

Onions appear like any other bulb, consisting of a leafy region at the top (which is often removed or dead before marketing); a thickened, layered, succulent region toward the bottom, which usually grows underground but is, in fact, a stem; and a basal plate at the bottom, from which fibrous roots grow. Green onions (scallions) and chives come from the leafy region; for most onions, garlic, and leeks, the edible portion (or at least the most commonly eaten portion) is the bulb.

History

The onion has a very disputed origin, as it is not easily preserved for archaeological purposes. Some say the onion first appeared in central Asia; others claim that Iran and Pakistan (further west) are more likely. It is known, however, that the onion has appeared in the Mediterranean region as far back as 5000 years ago, and that there are records of both onions and garlic in the Pentateuch, the earliest-written books of the Bible (reputedly). The Egyptians actually worshipped the onion as a symbol of eternal life, stemming from its "circle-within-a-circle structure"; onions were often left in the pyramids, and Pharaoh Ramses IV was mummified with onions in his eye sockets. In the 6th century BC, the Indians spoke of the value of onions as medicine, citing them as diuretics and good for digestion, the heart, eyes and joints. The Romans also enjoyed onions, often carrying them with them on journeys. In the Middle Ages, onions were used for everything from snakebite cures to rent payments. The Pilgrims carried onions to the Americas, only to find that the Native Americans already had their own wild onions.

Growing and Caring for Onions

Onions grow well throughout the continental United States, and in most temperate climates. There are certain varieties that only grow well in certain climates, though, so be sure to read packaging or accompanying literature carefully before planting. Onions are usually planted as soon as they can get into the ground; the length of the day affects bulb growth, so be sure to get them in the ground early enough that they can establish themselves before bulbing. Onions require frequent irrigation and weed maintenance, as they grow shallowly and compete poorly with other plants.

Harvest green onions anytime after the tops grow above 6 inches; larger green onions will be more pungent than smaller ones. Any bulb that has sent up a flower stalk should be harvested and used immediately; these do not store well. Allow the tops to fall over before pulling onions; harvesting too early results in small onions that do not store well. Instead, when the tops are sufficiently dry, pull bulbs in the morning and allow to dry till mid-afternoon (don't leave them out too long or that whole drying thing won't work). Then, you can braid the tops together or just raise the onions with a screen for a few weeks to cure them. After they've cured, you can cut the tops off and store in a cool, dry place with decent circulation. Onions will last for several months, but check them every once in a while and remove any that have gone soft or started to rot.

Purchasing and Storing Onions

Always buy onions that are firm, well colored (never black -- it's often a sign of rot), and that smell like onions (they pick up other scents and flavours fairly easily). Again, always store in a cool, dry place. They prefer to be in the dark, as light can discolor them and occasionally can encourage regrowth. Keep in mind that onions purchased in grocery stores have often been sitting around for a few months already, so try not to buy too many at once, intending to keep them forever; they'll either go bad or start growing on you. Onions can also be sliced and frozen, but I'd only use them for cooking afterwards, since freezing tends to cause things to lose some of their visual appeal.

Types of Onions and Their Uses

Onions have been cultivated in a number of ways, from the tiny pearl onion to the large, sweet Vidalias. The newer sweet varieties have been bred to reduce the sulfuric compounds present (which normally give raw onions that bite, and make your eyes sting). As such, they are much sweeter than their ordinary companions when eaten raw. However, cooking breaks down the sulfuric compounds, and as such, an ordinary Spanish onion may be much sweeter than a Vidalia when cooked.

White, yellow (Spanish) and red onions are the standard types of onions. These onions range from mild to sharp in flavour, and can be used for anything from salads to most cooked dishes to a delicate garnish for a dish.

Green onions (or scallions) are mild to pungent in flavour, depending on the size, and are often eaten raw or in salads, or lightly cooked in various dishes.

Leeks are very mild in flavour, and as such, accompany other vegetables or other dishes nicely. They make a wonderful soup, and are also excellent when braised in butter as a side dish.

Shallots are also mild in flavour, with a hint of garlic taste to them. They are excellent when used as a flavouring in meat and vegetable dishes.

Pearl onions are often sweet in flavour (though they can be quite strong, depending on the variety), and are often used in vegetable dishes and as a garnish. The onions used as cocktail garnishes are often pearl onions of a stronger variety to lend a stronger flavour to the drink.

There are dozens of hybrid varieties of sweet onions, including Bermuda, Maui Sweet, Walla Walla, Texas Sweets (or 1015s, named for the date they should be planted), Vidalias, Carzalia, Sweet Imperial, and so on. These onions are excellent eaten raw, or very lightly cooked to emphasize their sweet and mild taste.

Cooking Tips

Recipes Featuring Onions
Sources:

http://www.sunspiced.com/ohistory.html
http://www.onions-usa.org/ -- the National Onion Association
http://www.urbanext.uiuc.edu/veggies/onion1.html -- from the "Watch Your Garden Grow" online guide
http://www-ang.kfunigraz.ac.at/~katzer/engl/generic_frame.html?Alli_cep.html
http://www.ext.colostate.edu/pubs/garden/07614.html
How To Read a French Fry and Other Stories of Intriguing Kitchen Science by Russ Parsons
http://www.shallots.com