Soak ping pong ball sized tamarind in a cup of water for 15-20 minutes. If you like the rasam more sour increase the quantity of tamarind.
In a blending jar add tomato, cumin seeds, fenugreek seeds, onion, pepper corn, coriander, and grind to a paste. Keep aside.
In a heavy bottom wok heat oil and ghee.
Temper mustard seeds, cumin seeds, curry leaves, chopped shallots, dry red chilies, and asafoetida in the heated oil till you get the aroma of the condiments. Add the seasoning ingredients. Ensure the heat is completely reduced before you add the seasoning ingredients else you might burn the spices.
Add the ground paste to the sauteed spices and give a quick stir.
Now take the tamarind pulp and add it to the mixture in the wok. To get more out of the tamarind add more water and squeeze out the pulp with your hand. Strain and pour in the wok.
Let it boil.
Once the rasam comes to boil reduce heat and let it simmer for 2-3 minutes on low heat.