Bring four cups of water to boil and add whole tomatoes. Allow to boil for two minutes and switch off the stove. When it is cold, take out the tomatos from water and peel off the skin gently. Mash the tomatoes well (or put it in a mixie and run for one second).
In a kadai, pour the gingelly oil and when it is hot add mustard. When it pops up add jeera, fenugreek and asafotida. Fry till the fenugreek turns light brown. Add garlic flakes and fry for one minute. Then add sambar onion chopped along with curry leaves and fry nicely. Now add tamarind water, tomato puree, sambar powder, turmeric powder and salt and stir well. Allow to boil. Reduce the heat and cook on low flame till the kuzhambu is thick.