Pressure cook the dal in 1 cup water for 3-4 whistles. When cool, blend well with salt and turmeric powder.
Heat oil and temper with red chilies and mustard seeds. After it stops spluttering, add the chopped mango pieces and saute till light brown.
Add the boiled dal and simmer on low flame for 2-3 minutes. Add more water if you want a soup like consistency. Serve hot with steamed rice accompanied by fried potato strips.