Add soaked horsegram to pressure cooker with 1 ½ cups of water, after the first whistle, cook on low for 40 minutes till the horsegram turns soft & mashable. Let the cooker depressurise by itself.
Set aside 1 cup of cooked horsegram and add the rest of the cooked horsegram (approx. 1 ½ cups) to a mixer and blend to a thick paste using ¼ - ½ cup of cooked water for grinding.
Soak tamarind in 1 ½ tablespoons of hot water, rest for 10 minutes. Mash the tamarind well to get thick tamarind extract.
Heat oil in a kadai or cooking pot, add mustard, cumin, fenugreek seeds, hing, red chilli and once the seeds crackle, add garlic and saute for a few seconds. Add onions, green chilli, curry leaves and saute till onions turn translucent.
Add the horsegram ground paste along with 1 cup of cooked water, add kashmiri chilli powder, turmeric powder, salt, tamarind extract, jaggery, mix well and once it boils, simmer for 10 minutes for the horsegram paste to absorb all the flavours.
Add the rest of the cooked horsegram along with ½ cup of water, bring to a boil and simmer for 5 minutes.
For the tempering, heat oil in a tadka pan, add mustard seeds and once they splutter, add curry leaves, red chillies and saute for a few seconds till they are roasted. Switch off the heat, add kashmiri chilli powder, mix well, add to the dal and serve.