Method
For the rasam powder
Combine all the ingredients in a broad non-stick pan and sauté on a medium flame for 2 to 3 minutes. Allow to cool completely.
Once cooled, blend in a mixer to a smooth powder. Keep aside.

Combine the toovar dal with 1 cup of water in a pressure cooker and pressure cook for 3 whistles.
With help of a hand blender, blend the dal till smooth, add the prepared rasam powder into it, mix well and keep aside

Combine the tomatoes, tamarind pulp, turmeric powder, asafoetida, salt and 3 cups of water, mix well and cook on a medium flame for 8 to 10 minutes, while stirring occasionally.

Add the prepared dal-rasam powder mixture, mix well and cook on a medium flame for 3 to 4 minutes, while stirring occasionally. Keep aside.
For the tempering, heat the oil in a small pan and add the mustard seeds and curry leaves.

When the seeds crackle, add the tempering to the prepared rasam and simmer for 1 minute.
Add the coriander and mix well.