Devein the prawns and sprinkle with salt. Wash the coriander and pluck the leaves from the stalks. Finely mince the stalks with one green and one red chilli. Combine with the prawns. Now finely mince the onions, remaining chillis, garlic and 1 teaspoon coriander leaves. Brown the onions in some oil. Add the garlic, coriander leaves, curnmin, pepper and turmeric and fry for five minutes. Add the prawns, tomato purée and tamarind. Lower the heat and cook for five minutes. Mix flour with enough of the water to make a paste and add to the prawns. Stir in remaining water and cook to thicken, stirring continuously. Serve with rice.