Heat 1 tbsp oil in a saucepan, add the ingredients for tempering one after another in the same order. When the seeds splutter, add the onions and sauté on medium heat till they turn light golden.
Add the tomatoes, cover and cook till they get mushy. Add tamarind paste, 2 cups water and bring the mixture to a boil.
Stir in sambar powder, jaggery and simmer for 5 minutes.
Add the rice flour mixture, cook for another 2 minutes. Stir in chopped coriander leaves.