First step is to cook the lentils. This recipe calls for pink lentils also known as masoor dal. Pressure cook the lentils in turmeric + salt water along with ghee. Once cooked, mash it using a whisk and keep it aside.
If using fresh tamarind, then add it to a bowl of warm water and let it soak for 15 minutes. Then crush the soaked tamarind with fingers to extract the juice. Filter this juice into another vessel and repeat the same with the remains, 2-3 more times by adding them to fresh water and squeezing out the juice. But if you are using tamarind paste, add it along with the spices in the next step.
In a thick bottomed pan, add all the spices like salt, turmeric powder, rasam powder and asafoetida. Then add the tamarind juice, onions and tomatoes. Cook this on simmer for 7-8 minutes.
Add the mashed lentils along with curry leaves powder and chopped cilantro. Stir and turn off the flame at the sign of first boil. Actually fresh curry leaves are preferred, but if you don't have that, then you can use curry leaves powder.
Now prepare for the tadka. Heat ghee in a tadka pan and once it's hot, add mustard seeds, cumin seeds and slit green chillies. Once they crackle, take it off the flame and pour it over the rasam.
Serve it hot with some rice.