By Harold McGee
Onions are plants of the species Allium cepa, which originated in central Asia but has spread across the globe in hundreds of different varieties. There are two major categories of market onions in the United States, defined not by variety but by season and harvesting practice. Spring or short-day onions are planted as seedlings in the late fall, and harvested before they’re fully mature in the next spring and early summer. They’re relatively mild and moist and perishable, and best kept in the refrigerator. A special category of spring onion is the “sweet” onion— “mild” is more accurate—which is usually a standard yellow spring onion grown in sulfur-poor soils, and therefore endowed with half or less of the usual amounts of sulfur-containing defensive chemicals. The second major kind of market onion is the storage onion, grown through the summer and harvested when mature in the fall, rich in sulfur compounds, drier, and easily stored in cool conditions for several months.