Wash and soak ¼ cup tuvar dal for 1 hour.
Soak a marble sized tamarind in half a cup of water for 20 minutes. Mash the tamarind well, strain and keep tamarind pulp aside.
After 1 hour, drain and transfer soaked dal to a pressure cooker.
Add 3 cups of water and pressure cook for 4 whistles.
Open the pressure cooker once the steam has completely escaped. Whisk the cooked dal and keep aside.
Heat 1 tbsp oil in a pan.
Add mustard seeds, curry leaves, chopped green chilly, grated ginger, a pinch of hing and saute for few seconds.
Add chopped tomato and saute for few seconds.
Add tamarind pulp. Cover and cook till the tomato is cooked.
Add cooked tuvar dal and mix.
Add 2 tbsp rasam powder and mix.
Add 3 cups of water, salt and jaggery. Give a quick mix. Cook covered for few minutes.
Once done, garnish with chopped coriander leaves. Keep the rasam aside.