Heat a small skillet and dry roast red chilli, coriander seeds, fenugreek seeds, asafoetida in low flame, till you smell the aroma. Let it cool down and then grind it to a fine paste by adding water. Add tomato to it and grind it coarsely.
Take the juice from the tamarind by adding water.
Heat a vessel. Add mustard and urad dal. Allow mustard to splutter.
Add curry leaves and the grind paste. Allow it to boil for few mins.
Then add dal water, tamarind water and salt. Check its sourness. If its too sour you can add more water.
Bring it to boil and finally add chopped cilantro.
Your rasam is ready. Enjoy it with rice or just drink it as a soup.