The multilayered bulb known as the onion belongs to the same family as garlic and leeks. Onions are crisp and pungent when raw and become soft and sweet when cooked.
Two types of onions are available—fresh or spring onions and dried or storage onions. Fresh onions include green onions and sweet onions like Vidalia and Maui. Dried onions have been cured by drying, a process that causes their skins to tighten and protect against spoilage. The curing also concentrates their flavor, dries their skins and brings out the colors.
Choose fresh onions that look perky. For green onions, the shoots should be green with white bulb ends. For Vidalia and Maui, seek out fresh-looking, tubular green stems. For dried onions, choose firm specimens with smooth, dry skins. Avoid any with soft spots, particularly at the stem end; green shoots; moldy areas; or moist, wrinkled skins.
Store green onions in a perforated plastic bag for up to 2 weeks in the refrigerator. Keep other onions in a cool, dark, well-ventilated place. Do not leave them in plastic bags; instead, use a basket or crate to allow air circulation. Storing them alongside other vegetables (such as potatoes) that may give off gases or moisture will spoil the onions quickly. Discard onions that begin growing shoots, as they will taste bitter.
Cut all onions as close to cooking or serving time as possible. Their flavor deteriorates while their aroma intensifies over time.
To minimize the sulfurous odor (and tears), peel onions downward from the stem end and chop them with a sharp knife. Onions can be chopped in a food processor, but the pieces will be irregular; the blade also crushes the onion, releasing more juice and altering the flavor and texture.
When using green onions, trim away only the root end and any wilted or brown portions of the tops; both the green and white parts can be used, and recipes generally specify.
Adapted from Williams-Sonoma Kitchen Companion: The A to Z Guide to Everyday Cooking, Equipment and Ingredients (Time-Life Books, 2000)