Pressure cook the soaked lentils along with the chopped spring onion and turmeric powder in 1 & 1/2 cups water for 5-6 whistles.
Heat oil in a pan and temper with mustard seeds. After it stops spluttering, add the onion, curry leaves, ginger, garlic and asafoetida. Saute till light brown.
Add red chili powder, sambar powder and coriander powder. Mix everything well and add the tomatoes. Stir fry till it is mashed.
Now add the boiled dal and salt. Simmer on a low flame for 4-5 minutes. Switch off the flame and serve, garnished with coriander leaves.