Soak the tamarind in 1 cup water for 20 minutes.
Squeeze the pulp from the soaked tamarind with your palms in the water and keep aside
Chop the tomatoes and add this to the tamarind pulp
Make a semi coarse mixture in a dry grinder or in a mortar-pestle with the ingredients mentioned under rasam powder.
Heat oil in a pan and add the mustard seeds and cumin seeds and let them splutter
Add curry leaves and asafoetida and sauté for a minute
Add the tamarind pulp and crushed tomatoes mixture, grounded semi coarse rasam powder, Sambar masala
Stir and then add about 3 cups water, season with salt.
Bring to a boil and then simmer for 5-6 minutes.
Finally add chopped coriander leaves and give a nice stir.
Serve warm tomato rasam with rice papadam.
