Wash and pressure cook the dal with ½ cup water and a pinch of turmeric powder. Mash well adding about ½ cup of water and set aside.
Soak the tamarind in 2 cups of hot water and extract the tamarind juice. If using tamarind paste, dissolve in 2 cups warm water.
To the tamarind juice add tomatoes, garlic, turmeric powder, rasam powder, asafoetida, curry leaves and salt. Mix well and bring to boil.
Once it comes to boil, let it simmer for 6-8 minutes on low heat or until the tomatoes become soft.
Add the cooked dal, jaggery and mix well. Simmer till the rasam becomes frothy. Remove from heat. Add coriander leaves.
In a small pan, heat ghee and add mustard seeds. When it splutters, add red chilly, jeera and curry leaves. Pour over the rasam.