Pressure cook the toor dal with water upto its level and salt for one whistle or till done. Allow the pressure to go on its own.
Pour the onion, garlic, cumin and pepper in a mortar and pestle roughly.
In an earther pot, heat oil. Splutter the mustard and fry the dry chillies and curry leaves.
Add the pounded mixture and saute with a pinch of salt till nicely wilted.
Add the tomato and turmeric powder, sprinkle in some water - better the water wiping the mortar. Close and cook, stirring occassinally till the tomato is completely wilted.
Squeeze the tamarind completely and discard the tamarind. Pour the water into the rasam. Bring to boil.
Add the cooked dal along with its water, adjust seasoning and cook for another five minutes or till done.
Add the coriander leaves and switch off. Serve with rice and ghee, or have it as a soup along with a sadya.