Drain and cook the dal with 4 cups water and turmeric powder in the pressure cooker till 3 whistles are given out. Heat oil in a deep non-stick pan.
Add mustard seeds and let them splutter. Add red chillies, curry leaves, remove the pan from heat and add asafoetida. Put the pan back on heat, add onion and sauté well.
Dilute the tamarind pulp in a little water. Add radish to the pan and sauté for a minute. Add tomato and continue to sauté.
Add the tamarind water and a little plain water. Mix well and add sambhar powder.
When the mixture comes to a boil, reduce heat, cover and cook for 2-3 minutes or till the raw smells disappear. Add the dal and mix well. Once it comes to a boil, reduce heat and simmer for a while.
Mix rice flour with a little water and add to the sambhar. Mix and add salt. Cook till the sambhar thickens a little. Garnish with coriander leaves and serve hot with rice.