Heat oil in a pan and add the mustard seeds. When they pop, add the ginger, garlic and stir-fry for few seconds. Then add the onions and fry until soft. Add the tomatoes and mix well. Let it fry for couple of minutes. Now put the shrimp with red chili powder, turmeric & salt. Mix until the shrimp is well coated with the mixture. Add little water if the mixture sticks to the bottom of the pan. Lower the heat and cook for 7-10 minutes or until the shrimp is cooked.