Crush the ginger, garlic with mustard and cumin seeds. Set aside.
Heat oil in a pan and add the curry leaves and onions and fry until the onions are golden. Add the tomatoes, red chili powder and turmeric powder with 2 tablespoons of water ad cook until the tomatoes are soft. Then add the crushed spices and fry for 5 minutes. Now add the shrimp & salt and mix well. Pour 5 tablespoons of water and simmer for 10 minutes. Pour in the vinegar and cook until the sauce is thickened.