Pressure cook and mash toor dal. Reserve the dal cooked water.
Soak tamarind in 2 cups water for 15 minutes. Squeeze getly to extract pulp. Strain the tamarind water and add the dal cooked water to it.
To that , add turmeric powder, salt, curry leaves, asafoetida, crushed pepper cumin, chopped tomatoes and bring to 2-3 boils.
Add pressure cooked dal and simmer till the rasam turns frothy.
To make the tempering for rasam.
Heat 2 teaspoons ghee in a small pan.
Temper with mustard seeds, cumin seeds and curry leaves.
Add to the simmering rasam and mix well.
Garnish with coriander leaves. Serve rasam hot with rice.