Soak the yellow lentils in some water and keep aside.
Dry roast cumin, coriander, red chillies, and peppercorns. Let it cool.
In a mixer jar, take chopped tomatoes, tamarind, yellow lentils, ginger, garlic, and sufficient water. Blend to a fine paste.
To it, add coriander, cumin, and peppercorns, coconut and red chilies and blend. We want all these dry ingredients to be coarsely ground and not finely blended.
In a thick bottomed vessel, heat some oil.
Add cumin, mustard seeds and let them splutter. Add the red chillies and the curry leaves and sauté. Add asafoetida.
To this, add the blended paste. Mix well and cook for 5 mins.
Add salt and turmeric.
Now, add 300 ml of water to this and let it boil. Check the consistency and add some more water if needed.
Add the finely chopped coriander once the rasam starts boiling.
Boil for 15 mins on a slow flame.
Check the seasoning before turning off and adjust it by adding salt.