Not sure which recipe they use in Quebec. My friends eat "Moules frites" often im Quebec city. But,they seem very common in Montreal also.

I stand corrected, that it may indeed be Belgium in origin.

http://www.recipezaar.com/13498


http://www.epicurious.com/recipes/food/views/MUSSELS-AND-FRIES-WITH-MUSTARD-MAYONNAISE-241751

http://www.pierrejelenc.com/WIHFD/fish2.html