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 15-20 minutes.
Heat 2 tablespoons of oil in a pan, add 1 teaspoon mustard seeds, and let them splutter.
Add 1 finely chopped onion and 2-3 chopped green chilies, and sauté until the onions turn golden brown.
Add 1 tablespoon ginger-garlic paste and sauté for 2 minutes until the raw smell disappears.
Add 2 chopped tomatoes, 1/2 teaspoon turmeric powder, 1 teaspoon red chili powder, 1 teaspoon coriander powder, and salt to taste. Cook until the tomatoes become soft and the oil separates from the masala.
Add the marinated prawns to the masala and cook for 5-7 minutes, stirring occasionally.
Add 1/2 cup of water and cook for another 5 minutes until the prawns are cooked and the gravy thickens.
Garnish with chopped coriander leaves and serve hot with rice or roti.