Soak the tamarind in 2 cups of hot water for about 15 minutes. Squeeze the pulp well and extract the juice. Add 2 more cups of water to the squeezed pulp and extract that water as well.
In a heavy bottom pan, heat the oil. I use gingelly oil for the flavor, but feel free to use oil of your choice.
Add the mustard seeds and let it crackle. Then add the toor dal, red chilies and vendhayam (fenugreek seeeds) and fry for about 15 seconds.
Add the chopped onions and saute it for about 2 to 3 minutes until they start to become aromatic.
Now add the sambhar powder and give it a quick mix. Add the extracted tamarind water along with salt needed and curry leaves. Mix well and let it come to a boil.
Simmer the kuzhambu and let it cook for about 15 to 20 minutes.
Add the jaggery to the simmering kuzhambu at this stage.
Check the consistency of the kuzhambu. Depending on the quality of the tamarind, the kuzhambu could reach the desired thickness on its own, but sometimes we might need to add a little bit of rice flour emulsion to get it there.
If the kuzhambu is not to your desired consistency, then add a little rice flour which is mixed with about 4 tbsp of water to the boiling kuzhambu and mix well. The kuzhambu will thicken and at this stage turn off the flame.
Cover the pan with a lid to retain the flavor of the kuzhambu. Serve with hot rice and ghee!