Start out by cooking 3/4ths of a cup of mung lentils. This is easiest in a pressure cooker or Instant Pot, but you can also do this on the stovetop. To cook on the stovetop, add 1/2 tsp of turmeric to the lentils, cover with 2 inches of water, and bring to a boil. Cover the pot and continue cooking another 20 minutes or until the lentils are very tender and mushy.
Chop 5 medium green tomatoes into a dice of about half an inch.
Heat 1 tsp of vegetable oil. Add 1 tsp mustard seeds and when they start to sputter, add 1 tsp of the blackgram or urad dal, if using, a pinch of asafetida, if using, and 1/2 tsp fenugreek seeds, if using. If you’re not using the last three ingredients, just skip ahead and add the curry leaves next, saute them for a few seconds, then add the green tomatoes.
Stir everything well to mix, then turn the heat to medium-low, cover the saucepan and let the tomatoes cook about 5-10 minutes until they’re soft.
Add the cooked lentils or dal along with any remaining cooking water, add 1 tbsp of sambar powder and salt. Taste and add more sambar powder if needed -- I like about 2 tbsp in mine, but storebought brands vary widely from each other and from homemade versions.
Mix everything well and let the dal come to a boil. Add water at any stage if it starts to look too thick. When you add water, always check to see if you need more salt.
Let the dal continue to boil on low heat for another five minutes, then turn off the heat and add the cilantro.