Yep - as rascal says they are picked green. Then fermented - easiest is to buy Opies pickled walnut. Its funny but they don't really taste like walnuts but they do pack a umami punch. I always have a jar in the cupboard.Pickled walnuts are fantastic. I've always wondered how they are made.
People knock traditional British cooking because they are ignorant of what it represents. You bring the walnuts and a big wedge of Stilton. I will bring the Gala pork pies.