Boil 200 grams (3/4 cup) of toor-dal, chopped tomatoes, turmeric with 3 cups of water in a pressure cooker for 10 minutes (5 whistles). Let the pressure from the cooker release naturally. Set aside.
Heat sesame oil (Nalla ennai / gingelly oil ) in a pan until hot. Add in the mustard seeds, cumin seeds, fenugreek seeds and asafoetida. Let it splutter. Add in the small onions and saute till soft and translucent in medium flame. Roughly about 3-4 minutes.
Soak half a lime size tamarind in one cup of water for 20 minutes. Once the tamarind has been soaked, squeeze it between the fingers to release all the juices. Strain the juice. Set aside.
Once the onions are soft add in the spice powders and tamarind juice. Now let the tamarind mixture simmer for 15 minutes in medium flame. Simmering helps in cooking the masala powders and the onions.
In the meantime dry roast fresh shredded coconut until lightly brown. Grind it with ½ cup of water to a smooth paste. Set aside.
Now add in the cooked dal, coconut mixture and curry leaves. Adding curry leaves now to the sambar brings in a lot of fragrance and flavour to the sambar. Curry leaves are added at last for this onion sambar. Let it simmer for 5 minutes in low flame.
Switch off the flame and garnish with coriander leaves.
Serve hot with rice or any south Indian tiffin items.