Moules-frites is a traditional comfort food item consisting of mussels paired with Belgian fries on the side. The dish is commonly found in Belgium, the Netherlands, and northern France. The condiment of choice accompanying the dish is mayonnaise, providing an additional note of richness.
Mussels are a Belgian staple - cheap and abundant, they were originally considered a poor man's meal, and have been paired with fried potatoes for a long time at the country's famous friteries (fry shops). It is believed that the dish is originally from Belgium, because Belgians were the first to pair the mussels with fries, commonly eaten throughout the country in wintertime, when no fish was available.