Wash the toor dal, chop the tomatoes and coriander leaves, and measure out the rasam powder, turmeric, and tempering spices.

Soak the tamarind in warm water and let it sit until it softens and can be easily squeezed apart.

Boil the washed toor dal in water with a pinch of turmeric until the lentils turn completely soft and mushy.

Mash the boiled toor dal well with the back of a ladle or a whisk until mostly smooth, then set it aside.

Squeeze the softened tamarind in its soaking water to release the pulp, then strain out the fibers and seeds to get a smooth tamarind extract.

Combine the tamarind extract, chopped tomatoes, rasam powder, turmeric, and jaggery in a saucepan and simmer until the raw tamarind smell disappears and the tomatoes soften.

Stir the mashed dal and salt into the tamarind mixture, then let it simmer gently until small froth bubbles appear on the surface, and turn off the heat immediately after.

Heat the ghee in a small tempering pan, add the mustard seeds and let them splutter, then add the cumin seeds, dried red chilies, curry leaves, and asafoetida.

Pour the hot tempering over the rasam, stir in the chopped coriander leaves, and serve immediately while still hot.