Mustard Fish Curry is one of the most popular delicacies of eastern India, especially Bengal. Bengali recipes call for the frying of the main ingredient in mustard oil before it is added to the gravy or curry. Mustard Oil enhances flavors and causes the fish/meat/eggs to absorb the spices in the gravy better. In some Bengali homes, this frying translates to deep-frying, while in others it means pan-frying. Take your pick depending on your preference.