Wikipedia
The "true" pearl onion (A. ampeloprasum var. sectivum or A. ampeloprasum 'Pearl-Onion Group') is a close relative of the leek (A. ampeloprasum var. porrum), and may be distinguished from common onions by having only a single storage leaf, similar to cloves of garlic. They are cultivated mostly in Germany, the Netherlands, and Italy; usually in home gardens, although formerly on a commercial scale. They are mostly used for pickling.
However, the majority of onions grown for pickling are common onions (A. cepa). They are grown to a small size suitable for pickling by planting at a high density.
Because of its uniquely small size and a taste sweeter than that of a common onion, it has also been used in dishes ranging from mid-20th-century American casserole dishes such as succotash to sweetly flavored onion relishes in Indian cuisine. A pearl onion will take up to two years to become sufficiently sweet for sale (such is the main reason it is less popular in America than other onions). Pearl onions are, however, a staple to the cuisine of Northern Europe, especially Sweden.
Usage examples of "pearl onion".
Moored to its upper branches was a tiny pearl onion shape that, when Hamid-Jones's mind adjusted the scale, became a starship a mile in diameter.
A white pustle the size of a thumb tip had taken up residence on the left side of his snout, and it looked like a pearl onion in a turd.
Sol said, taking two chilled glasses from the refrigerator and, with the solemnity of a religious ritual, dropped a tiny pearl onion into each.