Wash the fish well and clean it from inside. Cut into darne. Sprinkle salt and half of the turmeric powder. Make a smooth paste of yellow mustard seeds, black mustard seeds, green chillies, ginger and salt.
Take a pressure cooker. Add three cups of water and place a ring in it.
Take a pan and place the fish in it and place the pan on the ring in the pressure cooker. Add remaining turmeric powder to prepared paste. Apply paste liberally on fish. Pour mustard oil on top. Adjust seasoning. Cover with lid and steam for eight to ten minutes.