Break up wadi into small pieces. Heat mustard oil in a large skillet or wok. Add wadi and fry until they are browned, about 2 minutes. Remove with a slotted spoon and set aside.
In the same skillet, add onion and ginger. Fry on low heat for about 10 minutes, until golden. Add cumin, coriander, turmeric and cayenne. Mix to combine. Add fenugreek and tomatoes. Mix well and cook for about 10 minutes, until tomatoes are very soft and mushy.
Add wadi, potatoes and 2 cups water. Bring to a boil. Reduce to a simmer and cook for 30 minutes. At this point, most of the water will have evaporated.
Add remaining water and cilantro. Stir to combine. Let it rest for 15 minutes.
Add salt and curry leaves. Stir to combine. Leave for 5 minutes for the curry leaves to infuse. Reheat for 5 minutes.