Stir the rice flour, turmeric and salt together in a bowl; add the prawns and turn in the flour mixture to evenly coat.
Heat 3 tablespoons oil in a large frying pan over medium heat; fry the cumin seeds in the hot oil until they splutter. Add the onions, green chillies, ginger and garlic paste; cook until the onions are browned, about 5 minutes. Stir the pureed tomato, chilli powder, garam masala and ground cumin into the mixture. Season with salt and continue cooking until the gravy thickens and the oil is released, 10 to 15 minutes. Pour the cream into the frying pan and stir; lie the prawns into the mixture and continue cooking until the prawns are cooked through, 3 to 5 minutes more. Garnish with the coriander to serve.