De-vein the prawns. Wash thoroughly and remove excess water. Add salt and keep aside. Grind the ginger, garlic, cumin seeds, red chillies, cloves, cinnamon and mustard seeds along with vinegar into a fine paste.
Heat the oil in a non stick pan and sauté the prawns till all the moisture dries up. Drain the prawns and set aside. In the same oil sauté the onions till they turn soft and light brown.
Add the tomatoes and sauté on high heat till they turn pulpy and the oil surfaces. Add the ground masala and sauté for two to three minutes. Add the prawns and sugar.
Check the seasoning and cook on low heat for five to seven minutes more or till the oil leaves the masala. Serve hot with pav or boiled rice.