Roast all ingredients mentioned for spice powder on a low flame. The mustard will pop, rice will turn golden brown. No oil is needed. Grind to a fine powder and keep aside.
To a kadai, add sesame oil. Add the mustard, red chillies and curry leaves. Then, add the shallots, garlic cloves. Cook until shallots turn translucent.
Add chopped tomatoes and salt. Let the tomatoes cook completely.
Add spices -chilli powder and coriander powder. Mix well and let it cook. At this stage, the oil begins to seperate.
Add 1.5 to 2 cups hot water and 1 tsp tamarind paste (if using raw tamarind, soak about raw tamarind and add as tamarind water). Check for salt and add if required. Let the tamarind and tomatoes cook together for 3-4 minutes.
If tomatoes are really tangy, then the tamarind can be even reduced but I felt this was right. I used regular truss tomatoes - not tangy ones.
Finally add the powder made and let it simmer in a low flame for only 2-3 minutes. The gravy will slightly thicken and oil will float on the top.
This is so tasty with rice/millets but also is delicious with dosai. It can be kept for 3-4 days in fridge. The flavour of mustard, pepper and coriander goes so well with a tangy kuzhambu!