We start by preparing the spice mix for the Mysore Rasam. Roast coriander seeds, asafoetida chunk, red chillies, peppercorns, on a low flame, in a few drops of ghee, until the chillies crisp up and other spices are aromatic.
Toast the chana dal (lentils) on a medium flame for 5 minutes or so, until golden.
Roast the dessicated coconut separately until golden and keep aside.
In the chutney attachment of the mixer or a small blender, grind this to a fine powder. This is your freshly prepared rasam powder for the Mysore Rasam. You can blend using some water to a fine paste, but this wont stay for long on refrigerating.
Make a tamarind pulp by soaking lemon sized tamarind in hot water for 15 minutes and squeezing out all the extract. Alternatively use 1-1.5 tsp of tamarind paste, whisked into 3/4 cup water.
Boil the tamarind puree along with the diced tomato for 5-7 minutes, until tomatoes are cooked. Add ground spice powder (all of it, to serve 6 people), 2 cups cooked tur dal, and 3.5 cups water. Add the salt and let it come to a simmer on a low flame.
Heat the ghee in a small saucepan for tempering. Add the mustard seeds, cumin seeds, when they splutter, add curry leaves, remove from flame and transfer over rasam. Remove rasam from heat and keep covered until ready to serve. Serve hot.