A pearl onion (Allium cepa), also called a boiler, baby onion, or button onion, is a small and round vegetable related to leeks and the bulb onion. This type of tree onion forms clusters of spherical and whitish bulbs that are often pickled when they are about one inch (two and half cm) or less in diameter. The edible bulbs can also be red, brown, or yellow in color and have a pungent odor and mild taste. These onions are cultivated around the world and used in American, Asian, and European cuisine.
There are multiple varieties of pearl onions. Most of them belong to the species Allium cepa, but are named and marketed according to the place where they are cultivated. Some pearl onions sold are just regular onion bulbs picked while they are still small enough to look like true pearl onions. A separate species of pearl onion, Allium ampeloprasum, has a bulb formed from a single leaf and can be found in Germany and the Netherlands. Another variety is the flat-shaped cepollini, cultivated in Italy, which has yellowish flesh and thin, paper-like skin.
Cultivation consists of planting the seeds tightly and densely, and then harvesting them when they reach the desired size. They are planted closely to purposefully stunt their growth. On average, seeds weighing a total of 80 to 110 pounds (36 to 50 kg) are planted in one acre (4,000 square meters) of land. Pearl onions can also be propagated by planting bulblets grown during the previous year. It will take almost two years for a pearl onion tree to produce bulbs sweet enough for sale.
During harvesting, pearl onions are undercut to separate the roots. The whole bed of the plant is hoisted, and then the bulbs are transferred in special conveyors designed for small onions. They are stored in open areas or cold storage in warehouses and arranged in shallow piles to minimize humidity, which will speed decay. At home, they can be stored at room temperature just before cooking.
Pearl onions are typically pickled and used as a condiment and garnish to make a cocktail onion. They are often served as side dishes and can be added to soups, sauces, and dishes like succotash and omelet. The tiny size and mild flavor of a pearl onion makes it ideal for preparing relish. It is best to boil the bulb and then cool it with ice water so that the skin can be peeled more easily.