To a pressure cooker add both the dals. Wash well & add 4 to 5 cups water. To this, add drumstick pieces & small onions. Add 1/4 tsp of turmeric powder and 1 teaspoon oil. Cook for 5 whistles or use your usual method to cook the lentils.
open the pressure cooker once the pressure subsides. The dal should be well cooked by now. Lightly mash and set aside.
Soak about a small lemon size tamarind in hoot water and extract about thick tamarind extract and set aside.
Heat 1 tbsp oil in a deep pan. Once the oil is hot enough, add in a few curry leaves. Once they splutter, add roughly chopped tomatoes and sauté until the raw smell of tomatoes goes away.
Now, add tamarind extract and jaggery. Sauté for a minute and add 1 cup of water. Allow this to boil till the raw smell of tamarind and jaggery disappears. This may take around 6 to 8 minutes.
Now, add the cooked dal mixture and mix everything together. cook for about 2 minutes and then add 3 cups of water. Adjust the consistency to suit your requirement. This sambar should be slightly runny as it will thicken once it cools down due to moong dal.
Add required salt & turmeric powder. Add 2 slit green chillies, a handful chopped fresh coriander and allow it to boil well (about 10 minutes).
Turn off the heat, add finely chopped Coriander and set aside while we prepare the tempering.