Heat oil in kadai and add cut tomatoes (big pieces). Once the tomatoes are well-cooked and the juice of the tomatoes thickens in the pan. Add tamarind paste, haldi and salt.
Cook for 10 more mins on medium heat.
Now the tomato mixture should be pretty viscous and hot. Cool it down by shutting off the stove and just letting it sit while you make the tadka.
For the tadka, heat oil add hing and dals. Saute them until they start turning golden. Then, add red chillies, green chillies and mustard (rai seeds).
Add garlic cloves if you like.
When you add green chillies to the tadka, cover it with a lid (a see through lid is preferable). Let the tadka cool down for 5-10 mins. Then grind the tadka first and add the tomato paste to the tadka paste. Whip for few seconds.
Add coriander as it is on top or whip it together and salt according to your taste. Serve with idlis, dosas or with just plain rice.