Pressure cook the toor dal with a pinch of turmeric powder, mash it well and keep it aside. Whenever you make sambar, you can cook some extra toor dal and keep it for making paruppu rasam the next day.
Make a puree of the tomatoes.
Coarsely crush whole peppercons, cumin seeds, red chili, coriander seeds and curry leaves. This is the instant rasam mix.
Heat oil/ ghee in a kadai, pop the mustard seeds, fenugreek seeds and curry leaves.
Add turmeric powder, asafoetida, prepared rasam mix and saute for a minute.
Add the tomato puree. Also add extra 1 cup of water and salt. If you prefer extra sourness, add some tamarind extract at this stage.
Boil this mix on medium flame and cook for 5-10 minutes till the raw smell of the tomatoes are completely gone.
Add the cooked toor dal, finely chopped coriander leaves and an additional 2 cups of water.
Cook till the rasam starts frothing. Do not boil the rasam as its taste will change.