Add oil in a hot pan and when the oil gets hot, add mustard seeds, cumin seeds, curry leaves, chopped green chillies, garlic, chopped onion and salt. Saute and fry the onions till they turn slightly golden in color. Add hing, turmeric, red chilli powder, coriander powder, and garam masala powder (optional). Mix well and add potato pieces. Saute the mixture and little water. Cover the pan and cook the potatoes until they are soft. Remove the lid and add chopped tomatoes, boiled black eyed peas and mix them thoroughly. Add water if required. Cook this for about 2 minutes or until it comes to a boil. Add finely ground fresh coconut and cook for a minute. Switch off the flame. Garnish with coriander leaves.