Soak tamarind in 1 cup of water and get the juice (discard the pulp).
Cook the pigeon pea by adding 3/4 of water, turmeric powder for 3 to 4 whistle. Once cooked and pressure release. Mash them well and keep it aside.
In cooking bowl add tamarind juice, tomatoes, rasam or sambar powder, turmeric powder, curry leaves and salt. Bring this to boil in low flame until the raw smell of tamarind is gone. (will take 15 to 20 minutes).
Now add the mashed pigeon pea, 1 1/4 cup of water. Cook this until the rasam stars to froth (forming) on the surface i.e. its about to boil. NOTE: don't boil the rasam.
In pan heat ghee. Add mustard seed when start to splutter add cumin seeds, red chili, curry leaves. When cumin start to sizzle pour this into the rasam and serve.