The potato onion (also known as multiplier onion) is a variety of the 'Aggregatum group' of Allium cepa, similar to the shallot, although producing larger bulbs. It is remarkably easy to grow, keeps better than almost any other variety of onion, and is ideal for the home gardener with restricted space. It was very popular in the past, but--like many old varieties--it has been passed over in favor of types more suitable for mechanical harvesting and mass marketing.
It is generally planted from bulbs, not from seed. Most sources say it should be planted in the fall, but this probably applies only to areas with moderate climates. It can be planted in the spring as early as the ground can be worked and produces well when so planted, up to at least the most northern limits of planting zone 5.
Sources differ about planting depth, some saying shallow planting is appropriate and others calling for deeper planting. This onion does tend to grow very close to the surface and a planting hole perhaps an inch deeper than the diameter of the bulb seems to work well. The onions vary in size from half an inch to three inches in diameter (1 - 8cm).
^James M. Stephens. "Onion, Potato". University of Florida, Institute of Food and Agricultural Sciences.