Put the prawns in a large bowl and sprinkle salt on them. Keep aside. Roast the chillies, cumin seeds, mustard seeds, peppercorns, turmeric, cloves and cinnamon till they begin to release their aroma. Remove from heat and cool. Grind the ginger, garlic and roasted spices into a smooth paste using the vinegar. Heat the oil on a medium flame in a wok-style pan to medium-high heat. Add the prawns and stir fry till opaque. Remove from the pan and keep warm. In the same pan, fry the onions till light brown. Add the tomato and curry leaves and fry until the tomatoes are soft. Now add the spice-vinegar paste, sugar and salt to taste and fry till the oil begins to separate. Add the prawns to this sauce, mix well and cook for 2-3 minutes. Serve with plain boiled rice and/or naan.