Heat oil in a kadai. Add mustard seeds and when they begin to crackle add green chilies; curry leaves, urad dal and sauté till the dal turns golden.
Add onions and sauté till translucent. Add ginger and continue to sauté. Add cashew nuts, turmeric powder and asafetida and stir well. Add potato pieces and salt.
While stirring mash the potato pieces lightly. Sprinkle a little water and continue to cook for further two to three minutes.
Serve hot with your choice of flat bread, dosa, or rice and dal.