Heat oil in a pan add mustard seeds. When it pops add the urad dhal and fry until golden. Add ginger, garlic and fry for few minutes. Then add the chopped onions and fry until it is translucent. Add the chopped tomatoes and mix well. Then add the potato pieces and green peas and fry for few minutes. Add red chili powder, turmeric powder and salt. Mix well and pour water until the potatoes are covered. Cover the pan. Lower the heat and cook until the potatoes becomes soft and the gravy becomes thick. Garnish with chopped cilantro leaves.