Wash and drain the dhal. Place the water, dhal, corriander, cumin, chillies and curry leaves in a pan and bring to boil. Lower the heat to medium and cook, uncovered, for 6-8 mins. Cover the pan and simmer for 30 mins.
Remove the pan from the heat and let cool slightly. Strain the liquid and sieve the dhal into it. Return to the pan. Heat and add the tamarind, salt, chillie powder. Stir until the tamrind is dissolved.
Heat the oil in a seperate pan and add the mustrad seeds. As soon as the seeds crackle, add the rasam or stir the hot oi and the seeds in to the rasam.