Pressure cook tur dal in enough water for 4 - 5 whistles till it is soft to the touch. You should be able to mash it when pressed between your fingers.
Keep the cooked dal aside and do not mash with a masher. We will be using it as such.
Place the drumstick pieces along with a cup of water in a bowl.
Add turmeric powder, 2 pinch salt and cook covered over medium flame for 3-4 mins or till the drumstick is fully cooked and soft. Keep aside.
Heat a pan with oil. Add Vengaya kari vadagam, urad dal and cook for few seconds.
Add onions, curry leaves, green chilies and saute till the onions turn translucent.
Add 1 cup water, sambar powder and cook for 2 mins over low flame till the raw smell disappears.
Add the cooked drumstick along with the water, add salt and cook covered for another 2 - 3 mins over low flame.
Add the cooked tur dal, mix well. Add 1/4 cup or more water, if the oorpu appears to be too thick.
Cook for 3-4 mins over low flame for the Murungakkai Oorpu to come together. It should be neither watery nor too thick.
Switch off stove and keep the pan closed for 2 mins before transferring to a serving bowl.
Serve hot with rice and a dry curry of your choice.