Heat oil in a kadai. Add mustard seeds to it. When they start to splutter, add urad dal and channa dal to it.
Fry for 2 - 3 minutes until it is fragrant.
Add onion and fry it until it is golden in colour. Add curry leaves, chopped coriander leaves, ginger and garlic and until it is light golden in colour.
Add turmeric powder, red chilli powder and coriander powder to it. Stir on a low heat for 1 - 2 minutes.
Add mashed potatoes and salt. Mix all the ingredients well. Do not add water for cooking. Cover it with a lid and cook on low heat for 8 - 10 minutes or until all the ingredients are well coated on potatoes.
Sprinkle chopped coriander leaves on it. Serve it with masala dosa.