Soak tamarind in a cup of water , when it become soft then mash and squeeze it and take out the pulp and keep aside.
Roughly chop the tomatoes and grind in a blender and make a smooth paste.
Add the tomato puree in a pan and add 3 cups of water ,chili powder and turmeric and boil on medium heat for 5 minutes.
Now add the tamarind water (1 cup) and salt in the pan and cook for 2-3 minutes.
Now add rasam powder and cook further for 3-4 minutes on medium heat.
Heat ghee in small pan and add the cumin seeds and mustard seeds. when they start crackling then add the dried red chilies , asafoetida and curry leaves.
Add this tadka on the boiling rasam and switch off the heat
Add chopped fresh coriander and cover the pan with a lid so it will absorb all the flavour. serve after 5 minutes.
Serve hot after with rice and ghee or as a soup