Heat the oil in a large, heavy-based pan over medium heat, add the asafoetida, mustard seeds, chillies, and curry leaves and fry for about 1 minute.
Add the potatoes together with all the ground spices, season with salt, then reduce the heat, cover, and cook over low heat for about 8-10 minutes, or until the potatoes are soft. Pour a little hot water into the pan to prevent the potatoes from sticking to the base (bottom) of the pan.
Put the gram flour and yoghurt into a bowl and mix together, then add this mixture to the potatoes.
Bring almost to the boil, then reduce the heat and simmer for about 7 minutes, or until the sauce becomes thick.