Pickled onion

Pickled onions are small onions which have been imersed in vinegar and pickled.

Incidents of Pickled Onions
The Fourth Doctor specifically requests a pickled onion from Rorvick whilst they sit in the Gateway, Romana also east a pickled onion. (DW: Warriors' Gate)

Mickey Smith has pickled onions (along with pickled eggs) in his cupboard, Jackie Tyler uses them to defeat a Slitheen (due to the vinegar content. (DW: World War III)

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