Take cleaned prawns in a bowl and add all the marination ingredients in it and mix well. Let it marinate for atleast 1 hour. The more marination the better.
Heat oil in a pan.
Add the mustard seeds and let it crackle.
Add the onions, green chillies and the half of the curry leaves and saute till onion becomes translucent.
Add the masalas except the garam masala powder. Mix well and saute for a min.
Add the tomatoes and fry till it turns mushy.
Add the marinated prawns, cover the pan and cook in low-medium flame till the water evaporates completely.
Add 1/2 cup of water, garam masala powder, remaining curry leaves, mix well and cook on high flame 5-10 mins and take off the flame.