Add the tomatoes, ginger, asafoetida powder, coconut, red chilli powder and salt. Grind together into a smooth paste and set aside.
Heat the oil and butter in a pan over medium heat. Add the mustard seeds, as soon as it starts popping, add the black gram dal and fry until it starts to turn golden. Add the methi seeds, fry for 10 seconds and add in the chopped onions, half of the curry leaves and saute until the onions turns golden.
Add the pureed tomato-coconut mixture and a tsp of salt to the onions stir well. Cook on medium heat for 10-15 minutes until the tomato masala cooks through. Mix in the chopped potatoes, cover and cook for 10-15 minutes until the potatoes are tender but not mushy.
Sprinkle rest of the curry leaves on top and serve with chapati or with steamed rice.