Wash the potatoes and pressure cook them in sufficient water & salt for about 5-6 whistles. Remove the cooker from the heat and let it cool to room temperature. Peel the potatoes and mash them coarsely. There is no need to mash them fine, a few lumps will give the raitha a good texture. Place the mashed potatoes in a serving bowl

In another bowl whisk the yogurt till smooth. Add enough water to give it a loose, creamy consistency. Add salt to taste and then pour this mixture into the bowl containing the mashed potatoes.

Heat ghee or oil a small pan for tempering and add the mustard seeds. When they stop spluttering, add the urad dal and stir until it turns golden. Toss in the hing and immediately pour this mixture over the potato raitha. Serve with pulaos or biryanis.