Pressure cook the dal with 4 cups water, turmeric powder, a pinch of asafoetida and ghee for 4-5 whistles. Blend along with the boiled mango and keep aside.
Grind the garlic, cumin seeds and peppercorns along with some water and keep aside. Heat 1 tbsp. oil and saute the ground paste till the oil separates.
Add the boiled dal, salt and sugar. Bring it to a boil and simmer on a low flame for 4-5 minutes. Switch off the flame and transfer it to a serving bowl.
Heat the remaining oil in a pan. Temper with dry red chilies, mustard seeds, curry leaves and remaining asafoetida.
After it stops spluttering. switch off the flame and pour this tempering over the prepared rasam. Garnish with coriander leaves and serve as a soup or with rice.