Pressure cook or cook dal with enough water and a pinch of turmeric in a pan. Mash the cooked dal well. Soak tamarind in water for 10 minutes and extract the pulp and strain.
Add the tamarind pulp to the dal with salt to taste. Allow to simmer on a low flame for sometime.
Heat 1tsp oil and season mustard. Add chopped onions, green chillies, tomato, turmeric powder, chilli powder, coriander powder, asafoetida and fry it for 2 seconds. Then mix the ingredients to the sambar and allow it to boil for 10 minutes. Remove from fire and serve hot. Garnish with chopped coriander leaves and curry leaves.
Note : Vegetables like drumstick, brinjal, potato etc can be added to sambar. Sambar powder or masala can be prepared earlier, stored and used whenever needed. 1 to 2 tsp can be used to make a curry.