Pickled onion

Pickled onions are a food item consisting of onions pickled in a solution of vinegar and salt, often with other preservatives and flavourings. There is a variety of small white pickled onions known as 'silverskin' onions; due to imperfections they are pickled instead of being wasted. They are frequently used as the distinguishing feature of a variation on the Martini cocktail, known as a Gibson. Pickled onions are pickled in vinegar and can vary in size. Silverskin onions are pickled in white vinegar, and are considered to be small. Full sized onions, e.g., Spanish onions, can be pickled if sliced first. In Hong Kong, pickled onions are served in many Cantonese restaurants, especially around dinner time, as a small dish before the main course is served.

Source: Wikipedia