Marinate the prawns with a tsp of lemon juice, a pinch of turmeric and chilli powders and salt. Meanwhile, heat oil in a thick bottomed pan, and stir-fry prawns until almost cooked. Drain and set aside. Into the same pan add cumin seeds, followed by crushed ginger and garlic and the thinly sliced onions and curry leaves. Saute until onions start to turn colour. Reduce heat and add the spice powders-turmeric, coriander and chilli. Fry briefly and throw in the chopped tomatoes. Add enough salt...