Grind the coconut, khas khas, sambar powder and tomato until it is a soft paste.
Add the ground paste to the onion lentil mixture. Add 200ml water and cook until the lentils are partially cooked.
Next, drain and add the chopped potato and aubergine (brinjal) and continue cooking until the lentils are soft and the vegetables are cooked.
Heat the remaining teaspoon of oil. Add the mustard and cumin seeds and curry leaves. Switch off heat when they sputter and the fragrant aroma is released.
Pour onto the cooked Kaal Saaru and mix the seasoning into the saaru /curry.