The traditional accompaniment to mussels is frites . According to historian Jo Gerard, the Belgians invented frites, way back in 1781. Frites are thicker than French fries and very crisp because, like traditional English chips, they're fried twice. You can pick up a cornet de frites from a mobile food stand known as frietkot. Frites special come with fried onions, otherwise try them with mayonnaise, tartar or béarnaise sauce or samourai, a spicy chilli mayonnaise