Rub turmeric, cayenne pepper & salt on halibut fillets, cover and place in refrigerator for 30 minutes.
Mix mustard powder, cayenne, turmeric & salt in 1/4 cup water and mix thoroughly.
Heat oil in medium frying pan on medium-high heat. Heat mustard, cumin & fennel seeds quickly until they pop, stirring quickly.
Pour in mustard sauce and stir.
Place fish pieces in the sauce and simmer gently for 5 minutes spooning sauce over the fish.