Pickled walnuts are a traditional English pickle made by pickling walnuts in malt vinegar. They are commonly served with cold meats and cheeses such as Stilton. There use in the United Kingdom dates back to before 1800, however there is some evidence that walnuts were pickled in the Persian area some 8000 years ago.
Charles Dickens mention pickled walnuts in The Pickwick Papers:
However, there he lay, and I have heard my uncle say, many a time, that the man said who picked him up that he was smiling as merrily as if he had tumbled out for a treat, and that after they had bled him, the first faint glimmerings of returning animation, were his jumping up in bed, bursting out into a loud laugh, kissing the young woman who held the basin, and demanding a mutton chop and a pickled walnut. He was very fond of pickled walnuts, gentlemen. He said he always found that, taken without vinegar, they relished the beer.