Take tamarind paste or tamarind water mix it in a cup of water put them in a cooking vessel or sauce pan.
Add salt, asafoetida and chopped tomato boil for 5 mins in medium flame.
Add blackpepper powder and cumin powder, water if required boil till tomato is done (approx 15 mins)
Now in another sauce pan heat ghee put cumin seeds let it sputter.
Add this to rasam garnish with corriander leaves.