Heat a heavy-bottom pan or kadai and add ghee. When the ghee melts, add the mustard seeds, cumin seeds, toor dal, dried red chili (broken into two), and the asafoetida.
Let the mustard seeds splutter, and then add the tomatoes, curry leaves, turmeric powder, and salt. Saute for two to three minutes or until the tomatoes are slightly soft.
Now add the tamarind water followed by rasam powder and ground pepper. Mix well. Bring this mixture to a gentle simmer over medium-low heat and allow the raw smell of the spices to go off.
Now add the remaining three cups of water, jaggery, and the cilantro.
Reduce the heat to low and cook until the rasam becomes frothy on top. Then turn off the heat.
Alternative method-Take a vessel in which you are going to make the rasam. Add the tamarind water, rasam powder, ground turmeric, ground pepper, salt, tomatoes, and curry leaves. Simmer this mixture for about 5 to 7 minutes. When it comes to a boil, add remaining water, jaggery, and cilantro. Let it simmer till it becomes frothy. Finally, do the tempering separately and add it to the rasam.
If you like your tomatoes mushy after the tomatoes turn soft, mash it with the spoon; if not, leave it a bit chunky.
I did not add any garlic. You can add two to three crushed garlic cloves along with tomatoes.
Adjust the salt and spices to taste.
You can add one tbsp of rasam powder and skip the ground pepper too.