Slit the onions and chop the garlic.
Finely chop the tomatoes and keep it ready.
Heat oil and temper with mustard, cumin, fenugreek seeds and hing.
Once it browns add the vathal and fry for 1-2 minutes.
Then add the onions and garlic and saute nicely.
Add the tomatoes if using and saute till mushy.
Then add red chilli powder, coriander powder and turmeric powder.
Add water and allow it to boil for 5 minutes.
Then add tamarind extract and allow it to boil for 10 minutes.
Then add powdered jaggery and allow it to boil.
Boil in low flame till oil separates.
Then add the thick coconut milk, give a quick stir and switch off the flame in one minute.
Delicious vatha kuzhambu is ready.
Serve it with piping hot rice/Idly/Dosa