Start by defrosting prawns. Transfer prawns to a bowl and add enough cold water to soak them properly for 15 minutes. Before using, wash the prawns once or twice in running water and add them to the curry.
Grind all the ingredients mentioned under ground masala to a smooth paste and set aside.
In a pan, heat coconut oil and add ginger and onions. Saute the onions till they are a light golden brown and add tomatoes. Cook the tomatoes for 5-7 minutes till they break down easily with the back of a spoon.
Add the ground masala and salt to the pan and bring it to a boil. Reduce the flame and simmer the masala for 10-15 minutes till the colour deepens slightly. Add the defrosted (drain the water) prawns to the gravy and cook them for 7-8 minutes. Sprinkle curry leaves on top, mix well and switch off the flame. Let the curry rest for 5 minutes before serving with steamed rice.