Dry roast toor dal, coriander seeds, cumin, red chillies and black peppercorn until fragrant.
Allow the spices to cool down and grind them to a coarse powder.
Wash toor dal multiple times with cold water and cook until soft with sufficient water. Alternatively cook in pressure cooker for 2 to 3 whistles or until soft.
Mash the dal and collect only the dal water in a container.
Place tomatoes in a deep bottomed sauce pan and mash them with potato masher.
Now add salt, turmeric, tamarind extract to the same pan and mix well.
Boil the mixture on medium high heat for 5 minutes or until raw aroma vanishes.
Now add water and simmer rasam for 10-12 minutes.
Reduce the flame to lowest possible setting, add toor dal water, ground spice blend and mix well.
Simmer rasam for 3-4 minutes or just until froth starts to form on the top.
Turn off the heat and add coriander.
For tadka, heat ghee in a second pan, add mustard and let it pop.
Finally add asafoetida, curry leaves and turn off the heat.
Add the tadka to rasam.
Serve piping hot rasam with steamed rice, pickle and papad or enjoy on its own as a soup