Mix the rice flour, salt, and turmeric powder in a large dish. Add the prawns and turn so all the prawns are coated with the flour mixture. The flour mixture will almost stick like a batter to the prawns.
Heat 3 tablespoons oil in a large frying pan or kadhai. Fry the prawns for just a few minutes. Do not overcook the prawns. You might need to cook them in batches. Keep aside for later.
Add a few more tablespoons oil to the frying pan. Add the cumin seeds. When they splutter, add the green chillis, onions, and ginger garlic paste. Fry till the onions are browned, about 5 minutes. Add some salt, cumin powder, and chilli powder. Now add the pureed tomatoes and cook till gravy becomes thick and oil is released, about 10 – 15 minutes. Add the prawns to the gravy, and cook just a few more minutes till the prawns are cooked through. Garnish with coriander leaves and serve.
Note: If you want the dish to be a little bit more rich, add ¼ cup cream when you add the prawns.