Clean and devein 500 grams of prawns, and marinate them with 1/2 teaspoon turmeric powder, 1/2 teaspoon red chili powder, and salt for 15 minutes.
Heat 2 tablespoons of oil in a pan, and shallow fry the marinated prawns for 2-3 minutes on each side. Remove from the pan and set aside.
In the same pan, add 1 tablespoon of oil, and sauté 1 finely chopped onion until golden brown.
Add 1 teaspoon ginger-garlic paste and sauté for 2 minutes.
Add 2 finely chopped tomatoes, 1/2 teaspoon turmeric powder, 1 teaspoon red chili powder, 1 teaspoon coriander powder, and salt to taste. Cook until the tomatoes are soft and the oil separates from the mixture.
Add 1 cup of water and bring the gravy to a boil.
Add the fried prawns to the gravy, cover the pan, and cook for 5-7 minutes on low heat.
Finally, add 1/2 cup of coconut milk and cook for another 2-3 minutes.
Garnish with chopped coriander leaves and serve hot with rice or roti.