Wash and pressure cook the dal with 1 ½ cup water and a pinch of turmeric powder for 3 whistles. Mash well and set aside.
Soak the tamarind in 1 cup warm water and extract the tamarind juice. If using tamarind paste, dissolve in 1 cup warm water.
Heat a kadai and add oil. Add the shallots and fry till they turn pink. Then add tomato and fry for 2-3 minutes.
Add drumstick, turmeric powder, sambar powder, salt and saute for 1-2 minutes. Then add ½ - ¾ cup water and cook the drumstick.
Once the drumstick is cooked, add the tamarind extract. Bring to boil and boil till raw smell of tamarind goes. Add dal, required water and simmer for for 4-5 minutes.
In a small pan, heat oil and add mustard seeds. When it crackles, add dry chillies, asafoetida and curry leaves. Pour over the sambar.
Notes
Make sure the drumstick is cooked well before adding the tamarind extract.
Instead of shallots, you can also use 1 medium onion, cut into quarters.