The Onion is a hardy, biennial native of western Asia. There are many varieties, which vary considerably in shape, coloring and size. They are commonly raised every year from seeds, either sown outdoors or inside, but they are often grown from small bulbs called Onion sets. These are raised from seeds sown in the summer of the previous year and if cultivated properly, will form good-sized bulbs by the end of the season. Onions grown this way, however, aren't as good as those from first-year seedling plants and they don't keep as well.

Onion sets are great for producing an early crop of Scallions. (These are young Onions that haven't developed a bulb yet, though the base may be swollen. Scallions are pulled and eaten in salads and as greens. The term is also given to young Shallots and Leeks.)

There are other types of Onion that are sometimes grown in gardens. One is the Potato Onion, which forms single bulbs or a cluster of bulbs beneath the soil.
The Top, Tree or Egyptian Onion is interesting in that it produces small bulbs at the tops of stems above the ground. The bulbs of these are usually used for pickling.
Another kind is the Welsh or Perennial Onion; it doesn't produce true bulbs, but instead is cultivated for the use of its stems and leaves, which are mainly used for flavoring.

Cebolla - Onion Link

Cebolla Properties Favored by Aztec's

Cures Measles,  Promotes and Enhances Sexual Performance,  Digestive,  Anti rheumatic,  useful for Diseases of the Respiratory Track (Asthma, Bronchitis, Cough, Colds, Laryngitis.),  It is of great use to people who drink great quantities of alcohol,  It is a Aperitif,  Cures Infectious Diseases,  for Varicose,  Cramps,  Neurological Pain,  Helps to avoid Apnea,  Eliminates Dandruff,  Cleans and Removes Spots and Blemishes on the Skin,  Helps the Body in Removing Abscesses and Tumors.

Aztec Method of Use