For preparing the delicious dish, heat oil in a large pan. Add the mustard seeds. As soon as they begin to sputter, add the onions, green chillies, ginger and curry leaves. Cook over medium heat until the onions are light brown.
Add the chilly powder, coriander powder and turmeric. Mix well and saute for another two minutes.
Add the prawns and the mangoes, stir in 1 cup water and bring the mixture to boil. Lower the heat and cook for 5 minutes.
Lower the heat under the pan, add the coconut milk and simmer for another 5 minutes, stirring constantly until the sauce thickens and the prawns are well cooked. Add salt to taste and serve hot.