Pickled onion

Pickled onions were small onions which had been immersed in vinegar.

Incidents of Pickled Onions
The Doctor specifically requested a pickled onion from Rorvick whilst they sat in the Gateway. Romana also ate a pickled onion. (DW: Warriors' Gate)

Mickey Smith had pickled onions (along with pickled eggs) in his cupboard. Jackie Tyler uses them to kill a Slitheen (due to the vinegar content and therefore the acetic acid which could dissolve a Raxacoricofallapatorian's calcium-based physiology). (DW:  World War Three)

Susan Mendes had a Gran on Earth who sent her pickled onions, she also left a jar of them for Alby Brook. (Invasion of the Daleks)