Clean vazhaithandu by taking out the excess strips around it by peeling off the thick ones. Slice off the thin outer ring (it will be opaque comparatively). Chop roughly into cubes and measure 1 cup.
Gather other ingredients.
Heat a small pressure cooker with coconut oil.
Add fennel seeds, ginger, garlic, chilli and fry for a minute.
In goes banana stem and little salt. Cook for one more minute.
Add 1 cup water and pressure cook for 3 whistles.
Once done, take out ginger, green chilli. Strain the ingredients for cooling down. Reserve the cooked water.
Once cooled, blend smoothly with little saved water.
Filter through a metal strainer.
Dissolve corn flour in little water and mix to the strained soup. Adjust salt.
Heat until it boils and continue simmering for a minute.