Pressure cook the rinsed toor dal for 3 whistles. Turn off the flame and wait until the pressure goes off. Mash the cooked dal and keep it aside.
Wash, peel and cook potatoes using a pressure cooker or microwave.
Heat 2 tsp of oil in a pan and add the coriander seeds, chana dal, rice and pepper and fry them on a medium flame till they are golden brown. Add red chillies to it and saute them for another minute.
Remove from the pan and set aside to cool.
In a mixie, add the roasted ingredients, roasted and tamarind and grind into a powder. Add coconut and grind it again. Grind everything to a smooth paste using water. Keep it aside.
Combine the cooked potatoes, ground masala, mashed dal, 1.5 cups of water (adjust to desired consistency), salt and turmeric powder in a vessel.
Let it boil on a medium flame for 10 minutes. Turn off the flame.
Heat 2 tsp of oil in a pan for seasoning. Add the mustard seeds and cumin seeds and let them splutter. Add the hing, curry leaves and red chilli and stir for a few seconds. Pour this over the stew and combine well.