Put the prawns, onion, green beans and asafoetida in a saucepan, add a little water and cook until the prawns and beans are done (if using fish, don't add it yet).
Meanwhile, grind or blend the coconut, chillies, turmeric, pepper and tamarind into a thick, fine paste, adding only as much water as you really need.
Stir the paste into the prawns and bring to the boil (add the fish now, if using, and cook for 5 minutes).