Cook the potatoes in the cooker till soft. Cut mushrooms in small size.
Heat oil in a big pan. When it is hot, add mustard. When it bursts, add chopped onion, tomato, curry leaves. Stir till it is soft. Add all the masala powders. Stir till the raw smell goes. Add the mashed potatoes to the pan and also the mushrooms. Stir well. Add enough salt and water.
Grind the coconut and green chilly. When the curry is hot, add this mixture to the curry. Stir well. Cover the pan and wait till it boils and is cooked.