Start by preparing the banana stem. Bring the water to a boil and add the chopped banana stem pieces. Cook them until they become a little mashed. Make sure to use the soft part of the stem and discard the outer layer and hard parts.
For the curry powder, dry roast all the whole spices and let them cool down to room temperature. Once they cooldown, grind them into powder. Keep aside.
Next, heat oil in a pan. Add the mustard seeds, cumin seeds, curry leaves, and dry red chilies.
Add the onions next and saute them until they become translucent.
Add the boiled banana stem next and saute for 2 minutes.
Add all the powdered spices along with the prepared curry powder and salt and mix well. Let it cook for 1 minute.
Add the curd and the gram flour and mix well. Let it cook for another minute.
Garnish with chopped coriander leaves and serve hot with sambar and steamed rice.