Devein the prawns and keep the heads and tail intact. Grind the coconut with water to a fine paste.
Bring one forth cup water to boil in a non-stick pan. Add mustard paste, turmeric powder, green chillies, salt, sugar, mustard oil, cream and coconut and mix well.
Add the prawns and mix well. Remove and pour the prawns with the mixture in a stainless steel tiffin box.
Keep the box in a pan. Fill the pan to the half quantity of the box. Lower the heat and cook for thirty minutes.
Remove and rest for ten minutes.Serve with steamed rice.