Clean Prawns and drain the water completely from the prawns.
Heat 2 tbsp of oil in a large skillet and add the chopped onion and 3/4 tsp salt. Cook over medium heat, stirring sometimes until the onion is tender and light golden in color. It takes about 10 minutes.
Add green chilies and saute for 30 sec. Add the ginger-garlic paste and cook until fragrant, about 2-3 minutes
Add chopped tomatoes and mix, stir frequently until tomatoe is soft, about 5 minutes
Reduce heat to low and add chili powder, turmeric powder, cumin powder, coriander powder and cook for 2-3 minutes, stirring frequently until spices are fragrant. Add 1/4 cup water. Saute the masala for 2-3 minutes over medium heat.
Add prawns. Mix well and cook shrimp for 5 minutes stirring often over medium heat. Add 1 cup coconut milk. When it comes to a boil cover it with a lid. Cook for another 10 minutes on low heat.
Remove lid and add 1/4 tsp garam masala. Mix very well and remove it from the heat. Garnish with coriander leaves. Serve with Rice, Roti etc.