Heat oil in a kadai, add onions and curry leaves. Saute till light brown.
Add green chilies and tomatoes, mix well and cook till tomatoes get soft. Add salt, turmeric powder, red chilli powder, coriander powder and tomato sauce. Add ¼ cup water and mix well.
Add the prawns and mix well. Let the mixture boil and allow it to thicken. Switch off the gas. Allow it to cool.Make a dough with rice flour, salt and little water. Divide the dough into equal portions and shape to form small balls.
Flatten each ball and stuff a spoonful of the prawn gravy in the center and roll again to form small balls. Coat the balls with grated coconut.
Heat water in a steamer and steam the balls for ten minutes or till rice flour is cooked.Serve hot.