The name "scallion" is applied to several members of the onion family, including a distinct variety called scallion, immature onions (commonly called green onions), young leeks and sometimes the tops of young shallots. In each case, the vegetable has a white base that has not fully developed into a bulb and green leaves that are long and straight. Both parts are edible. True scallions are generally identified by the fact that the sides of the base are straight, whereas the others are usually slightly curved, showing the beginnings of a bulb. All can be used interchangeably although true scallions have a milder flavor than immature onions. Scallions are available year-round but are at their peak during spring and summer. Choose those with crisp, bright green tops and a firm white base. Store, wrapped in a plastic bag, in the vegetable crisper section of the refrigerator for up to five days. Scallions can be cooked whole as a vegetable much as you would a leek. They can also be chopped and used in salads, soups and a multitude of other dishes for flavor.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.