Normally banana stem comes with 2 layers, the outer porous one and the inner hard edible portion. Make a slit and cut open the outer porous layer. The inner hard core is used for cooking. If you get the inner core as such, then avoid the step above.
Slice into circles, while doing so you can find lots of fibers from the stem. Remove it using fingers and continue chopping. Later chop them into small pieces and immerse in water
There will be fibers still present even though some was removed during slicing. To remove that, run a rod or wooden stick into the vessel in a circular pattern.
Once you pull out the rod/stick you can see fibers attached to it. Discard it and repeat the process a couple more times.
Wash green gram and cook it for a whistle by adding enough water.(can even add sprouted green gram for more healthier version) Grind coconut, red chilli, garlic and cumin to a slight coarse paste.
Add banana stem, grinded coconut mix, salt and turmeric powder to green gram and cook for a whistle. (No need to add water if there is remains after cooking green gram, else add quarter cup water)
In another pan add oil, splutter mustard, curry leaves, onion and saute. Garnish it.
Banana stem lentil sabzi is ready to serve with rice.