Marinate the prawns with the salt, green chillies and onion for half an hour.
Grind all the other ingredients except tamarind pulp and 2cups water. Mix the ground masala with 2cups of water in a pan and boil on medium flame. When the mixture reduces to half of its quantity, add marinated prawns and tamarind pulp. Add salt if needed. Stir well and cook for 10 minutes.