- Grind all red chilli,cumin, mustard, turmeric,ginger,garlic paste using 2 tbsp. vinegar and a little water to a smooth paste. Rub the prawns with 1 tsp. salt and the ground masala paste and marinate in the refrigerator for 1 to 2 hours. Later, heat the oil in a pan and sauté the onions for 2 minutes on low heat till soft. Add the marinated prawns and sauté till they change colour. Next, add water, the boiled and chopped potatoes, 1 tbsp. vinegar, ½ tsp. salt and the sugar and cook covered on low heat for 5 minutes. After 5 minutes, add the grated tomatoes and cook covered on low heat for another 5 minutes. Check for salt and vinegar and add if required. Cook for a further 5 minutes. Serve hot.