Heat oil in a pan. Add mustard seeds, let it splutter. Add cumin seeds and sauté for few seconds. Add asafoetida, curry leaves, ginger and green chilies. Sauté for few seconds. Add sliced onion and sauté until it turns translucent. Add red chili powder, turmeric powder and salt to taste. Sauté for few seconds. Add water and mashed potato, mix well. Add lemon juice and coriander leaves. Add more salt if required, mix well and cook for a minute or two.