Soak the tamarind in 2 cups of hot water for 10 minutes. strain it and keep the tamarind water aside. Grind the black peppers, cumin seeds, garlic and tomatoes and make it into a paste. Add this to the tamarind extract. Then add crushed red chilies, coriander powder, rasam powder and steamed salt to it and mix well. Set aside.
Heat the oil in a pan and add mustard seeds. when it pops add the urad dhal and fry until golden. Add asafetida and the curry leaves. Remove the pan from heat and pour this seasoning over the tamarind extract mixture. Add the steamed thoor dhal to it. Heat this mixture. When it starts to boil remove from heat. Garnish with chopped coriander leaves.