Pickled onions were a prepared food found on Earth. (TV: World War Three, AUDIO: Invasion of the Daleks)
The Fourth Doctor specifically requested a pickled onion from Rorvik whilst they sat in the Gateway. Romana II also ate a pickled onion. (TV: Warriors' Gate)
Mickey Smith had pickled onions (along with pickled eggs and gherkins) in his cupboard. Jackie Tyler used them to kill a Slitheen. The vinegar (and therefore the acetic acid) they were pickled in could dissolve a Raxacoricofallapatorian's calcium-based physiology. (TV: 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. (AUDIO: Invasion of the Daleks)