(most popular festive Bengali style steamed Hilsa with mustard paste and mustard oil. )
Soak the mustard seeds in some warm water for about 20-30 minutes before grinding them. Marinate the fish pieces in salt. Grind the mustard seeds, 2 green chilies and coconut to a fine paste. Remember to add some salt while grinding the mustard. Add water as needed to make it a smooth fine paste. The mustard gets a bitter taste if you grind without the salt.
Spread the fish out in a single layer in the pressure cooker separator containers. Mix a pinch of turmeric with this paste and pour over the fish such that the pieces are all coated in the paste. The paste should not be very watery. Pour a generous helping of mustard oil on the fish and paste. Put in the pressure cooker and steam for 2 or 3 whistles.