Soak the horsegram overnight in water or atleast a minimum of 8 hours. Next day, drain all the soaked water from the dal.
Take horse gram in a pressure cooker, add enough water, baking soda,salt and turmeric powder to the dal. Pressure cook for 4-5 whistles or until soft.
While the horsegram is cooking, soak cashewnuts in hot water for 15 minutes.
Let the pressure release on its own. Once done, drain the water. keep the dal aside.
Now let us prepare the masala. In a mixer, grind together coconut, soaked cashewnuts, chilies, ginger and coriander. Add ¼ cup of water and grind this to a smooth paste. If you need more water, add another 1 or 2 tablespoon not more than that.
Heat a pan with coconut oil, splutter mustard seeds, crackle cumin seeds, add curry leaves and red chilies. Add chopped tomatoes to this and saute until they turn mushy.
Now add the ground masala to this and the cooked horsegram dal. Add ½ cup water to this and salt. Let this simmer for few minutes, until the masala flavors blends well with the dal and the curry thickens.
Once done, garnish with fresh corinader leaves and serve warm with steaming hot rice or alongside roti.