Heat the oil or ghee in a large, heavy-based pan over medium heat, add the asafoetida, mustard seeds, dried red chillies and curry leaves and stir-fry for about 1 minute.
Add the potatoes together with all the ground spices, season with salt, then cover, reduce the heat to low and cook for about 10-15 minutes, or until the potatoes are soft.
Pour a little water on the lid and tilt it into the pan when hot to prevent the potatoes from sticking to the base (bottom) of the pan.
Remove the pan from the heat, add the yoghurt slowly, stirring constantly, then return to the heat and simmer for about 5 minutes, or until the sauce is thick, but do not allow to boil.