Blend the banana stem, tomato, coriander stems and chili along with some water into a smooth paste. Strain it and add more water according to the consistency you prefer.
Transfer to a kadai / pan and add asafoetida, salt, turmeric powder and rasam powder. Bring it to a boil and simmer for 2-3 minutes.
Switch off the flame and add the lime juice. Transfer to a serving dish. Heat oil / ghee in a pan. Temper with mustard seeds and red chili.
After it stops spluttering, add the curry leaves. Saute for a few seconds and switch off the flame.
Pour this tempering on the prepared rasam / soup and garnish with coriander leaves. Serve in individual bowls and relish as a soup. It can also be had with plain steamed rice and some side dish or papad.
