First wash and soak the mochai overnight. Then pressure cook them along with a pinch of salt and asafoetida by adding about a cup of water for 4 whistles or until soft. Keep aside.
Soak the tamarind and extract half a cup of tamarind juice by adding required water. Keep aside.
Heat a pan with ¼ tsp of oil and add all the ingredients given under "To grind", except coconut and fennel seeds. Saute for 2-3 mins until the onions start becoming translucent. Transfer them to the plate, let them cool down to room temperature and then grind them in a mixie along with coconut and fennel seeds to a fine paste. Keep aside.
Now heat a pan with oil, add the mustard and once the mustard splutters, add the other ingredients listed under "To temper" in the given order. once the broken urad dal starts turning golden brown, add the curry leaves, garlic, small onion and saute until the onion turns translucent.
Now add the tomato, brinjal, drumstick and saute for 2 minutes. Add a cup of water and give a quick mix. Now add all the powders (turmeric, sambar, asafoetida, and salt) and give a quick stir.
Add the prepared ground paste along with a cup of water and give a stir. Let the kuzhambu come to a boil. Once it starts to boil turn the flame to low-medium and let the gravy keep cooking until the veggies are ¾ done.
Now add the tamarind extract and continue cooking until the raw smell of tamarind is gone and veggies are almost done. Now add the cooked mochai (along with the water used while cooking them) into the gravy and cook until the desired consistency is reached. Turn off the flame.
Enjoy hot with rice/idli/dosa or chappati.