Soak tamarind in hot water for 15 minutes and extract its juice. Discard the pulp
Soak toor dal in 1 cup of water for 15 minutes.(Soaking helps the tur dal to get cooked well and also saves time and fuel.) Pressure cook tur dal for 3 whistles or until soft. Mash tur dal well and keep it aside.
Heat a tsp of oil and fry coriander seeds, bengal gram dal, red chillies and raw rice. Fry on medium heat until dal turns golden brown. Add grated coconut in the end.
After adding coconut, switch off the flame. Saute the coconut in the heat of the pan itself. Leave it to cool and then grind it to a smooth paste. This is the arachu vitta sambar masala.
Wash radish, peel the skin and cut it into round shapes. Cook radish also along with tur dal keeping it in a small vessel along with dal.
Heat 2 tsp of oil in a pan or kadai, add mustard, when it splutters, add fenugreek, hing and curry leaves.
Add chopped onions (if using) and sauté for a few minutes. Since I have used radish I pressure cooked along with tur dal. In case, if you are using vegetables like ladies finger, brinjal, pumpkin, drumsticks, you have to sauté it in oil along with onions.
Then add 1 1/2 cups (approximately) of tamarind extract, turmeric powder, salt and boil until the raw flavor of the tamarind goes.
Then add the ground paste (arachu vitta sambar masala), cooked dal, cooked radish along with a cup of water and boil for a few minutes on medium flame. The sambar should neither be too thick nor too watery, so adjust the consistency of the sambar accordingly
Garnish with coriander leaves and serve with rice or idli or dosa or ven pongal.