Clean prawns and marinate with half the turmeric powder and salt for 10 minutes.
Heat mustard oil and lightly fry prawns for 1 minute on each side; remove.
Add grated onion, ginger paste, and garlic paste to oil; sauté until light brown.
Add remaining turmeric, salt, slit green chilies, and coconut milk.
Simmer gravy for 5 minutes, add fried prawns, cook for 3 minutes, and finish with garam masala.