Boil toor dal in 1 1/4 cup water with turmeric and salt until soft and mushy.
Add coriander powder, chopped tomatoes, chili powder, methi powder, hing, and tamarind paste to the cooked dal.
Simmer the mixture for 5 minutes, stirring occasionally.
Heat 1 tablespoon oil in a frying pan over medium heat.
Add mustard seeds and let them pop for 30 seconds.
Add chopped onions and fry for 5 minutes until golden brown.
Add curry leaves to the tempering and fry for another minute.
Pour the tempering into the dal mixture and stir once.
Serve hot with idlis or white boiled rice.