Warm oil. Season with mustard , chana dal and curry powder. Saut peeled onions till translucent. Add squeezed out extract of tamarind. Throw the sambar powder, salt and hing in. Let simmer till onions are cooked. Add dissolved rice flour water for thickening. Stir and wait till it simmers again a couple of minutes. Remove.
Variations: Tomatoes with chick peas makes a delicious vatha kozhambu. Add some slit green chillies if you would like to spike it up. Other suggestions are diced sweet potato, diced butternut squash, daikon (Chinese radish), drumsticks (available fresh in Indian stores or canned, pre-cooked) all on their own.