Soak tamarind in hot water for ½ hour and extract tamarind juice.
Slice off the skin of the yam and cut into small, thin squares.
Heat the tamarind extract with more water (total 4 & ½ cups) and add the chopped yam to it. Let it get cooked atleast half.
Add sambar powder, turmeric, salt and red chilli powder (if adding) to the boiling kuzhambu.
Mean while, heat a kadai with oil and temper with fenugreek, mustard, urad dal, toor dal, cumin seeds and curry leaves in order. Add roughly chopped onion and garlic to it and fry for 2 minutes in medium flame.
Add it to the boiling kuzhambu. let it boil until the vegetable gets cooked properly and the volume reduces.
Lastly add jaggery, pepper powder. Once thick, add sesame oil and switch off the stove.