Cook thuar dal and keep aside. Soak tamarind in hot water for atleast 10 minutes. Squeeze and take out the tamarind water alone. Add salt, turmeric powder, hing powder and few fenugreek seeds to this and boil it. After the tamarind smell disappears add the cooked vegetables. Fry channa dal, red chillies, remaining fenugreek seeds, coriander seeds in oil. Remove them and in the same vessel add coconut powder and fry them till pink. Grind these together to a fine paste and add to the boiling solution after 5 minutes. When you get a good aroma, add tuar dal to this solution. Heat oil and saute green chillies and mustard seeds and add them to the boiling sambar solution along with coriander and curry leaves.
Note: If vegetables like lady's finger, brinjal, carrot are used, you can add directly to the boiling tamarind solution without cooking before. It will get cooked in the tamarind solution itself. But if Potatoes, colacasia and all roots are used it should be cooked previously.