Heat oil in a kadai. Add mustard seeds and when they begin to crackle add green chillies, curry leaves, urad dal and sauté till the dal turns golden.
Add onions and sauté till translucent. Add ginger and continue to sauté. Add cashewnuts, turmeric powder and asafoetida 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.
Place this bhaji in the centre of a dosa, roll in both the sides and serve hot with chutney and sambhar.