Boil the potatoes until they are tender, then peel and slice them into quarter inch-thick round disks.
Heat oil in a saucepan. Test the heat by adding a cumin seed; if it cracks, the oil is ready.
Add black mustard seeds, cumin seeds, and asafetida. Once the seeds crackle, add red chili powder, coriander, fennel powder, and turmeric.
Stir briefly, then add chopped tomatoes and green chilies. Let the mixture simmer for 2-3 minutes.
Add the potato slices.
Cover the pot and cook for about 10 minutes over medium-low heat. Add sugar and cilantro. If the gravy becomes too thick, add water as needed. Cover and let it sit for a few minutes.
Adjust the salt and sugar to your taste.