Boil potatoes in the skin until cooked. Peel, coarsely mash, and set aside.
Heat oil in a wok over medium-high heat. Add mustard, cumin, bengal gram, asafoetida, and turmeric, stirring. Add green chilies and ginger. Cook 2 minutes more, then add onions and curry leaves. Cook until onions are soft. Add mashed potatoes and salt, to taste. Mix well. Add water, mixing well again. Stir in chopped cilantro. Serve with dosa.