Wash and pressure-cook dal in two and a half cups of water with turmeric powder and one teaspoon of oil. Mash the cooked dal lightly with a wooden spoon. Soak tamarind in one cup of warm water. Remove the pulp, strain and keep aside. Wash, remove stems and slit green chillies.
Peel and wash sambar onions. Clean, wash and chop coriander leaves. Wash and pat dry curry leaves. Heat remaining oil in a thick-bottomed pan, add mustard seeds, stir-fry till they start to crackle. Add whole red chillies, fenugreek seeds and asafoetida.
Add slit green chillies, curry leaves and sambar onions and cook on medium heat stirring continuously for four to five minutes. Stir in tamarind pulp, sambar powder, salt and one cup of water. Reduce heat and simmer for six to eight minutes or till onions are tender.
Add boiled dal and simmer for two to three minutes. Blend rice flour in a quarter cup of water and add to sambar. Stir well and cook further for two to three minutes, stirring occasionally. Sprinkle chopped coriander leaves and serve hot.