Wash and boil the potatoes in a cooker. Peal off the skin.
Chop the tomatoes to fine pieces. Slit the green chilies in the middle. Keep aside. Finely chop the coriander leaves.
Heat oil in a pan. Once the oil is hot, add mustard seeds, cumin seeds, asafetida and turmeric powder. Once it starts spluttering, add chopped tomatoes. Sim for 5 mins.
Add mashed potatoes and required quantity of water. Mix it well and bring it to a boil.
Since I didn't have Black maharashtrian masala, I added the regular garam masala that I had, then salt and half of the coriander leaves.
The dish is ready within 5 mins.
Garnish with the remaining coriander leaves.
Notes
I served this with hot puris and it was really good. My daughter and athamma wanted this to be included in our regulars. This is a variation for us, as we never make potato gravy without onions.