Wash dal and pressure cook with required water and turmeric powder for 3 whistles or till cooked. Open lid once pressure drops.
Soak tamarind in 1/2 cup warm water and extract the juice. Strain and discard the fibres.
Slit green chillies, finely chop tomatoes. Chop onion. Cut the drumsticks into 3" long pieces.
In a saucepan pour in the tamarind extract. Boil for 5 mins. Add green chillies, tomatoes. Add about a cup or two water and bring to boil once again.
Add cooked dal, rasam powder, drumstick, salt. Simmer for 15 to 20 mins.
Add jiggery, coriander leaves.
For the tempering, heat ghee. Add mustard. Let crackle. Add curry leaves, asafoetida, onion. Saute till onion turns transparent. Do not brown the onion. Pour over the rasam.
Stir and serve piping hot with rice and papad!
NoteAdding onions to rasam is optional. But they give a nice flavour along with the drumstick.
You could substitute ghee with cooking oil.
You can add a tablespoon or two of grated coconut to the rasam along with coriander leaves.
Adjust consistency by adding water as required.