To prepare the banana stem curry, first step is to chop the banana stem. Oil hands before handling the stem as it stains the hands.
Peel off the satiny outer layers with a knife until the inner rough part is exposed. This could be 2 layers or more depending on how thick the stem was to start with.
Keep a big bowl of diluted, sour buttermilk on the side to immerse the chopped stem pieces. This prevents them from darkening.
To cut the plantain stem, slice it thinly - 4-5 slices at a time, place them in a stack and then finely dice it, immersing the chopped pieces in the buttermilk immediately. Do this until you have chopped the entire length of the stem into small bits. Any tough fibres that come in your way, peel them off and discard. Small fibre bits will remain and that's what will make this dish extremely healthy.
Once you are ready to cook, drain the chopped plantain stem and place in a vessel with 1/4 cup water and turmeric powder mixed well.
Place 2-3 cups of water in a pressure cooker large enough to hold the vessel containing the chopped stem.
Pressure cook this for 3 whistles and 3-4 minutes on sim. Once cool, remove and drain if required. You can also boil this in very little water until almost soft.
In a kadai, heat the oil, saute the udad dal till light golden, put in the remaining tempering ingredients (asafoetida, curry leaves, mustard seeds, dried chilies). After a quick stir, put in the drained moong dal and cooked plantain pith. Add sugar and salt, plus sambar powder if using. The pinch of sugar is to offset the slight bitterness in the stem.
Sprinkle water and cook covered for 10 minutes with occasional stirring, so that the dal is cooked but not mushy.
Check for salt and adjust, remove from flame. Sprinkle fresh coconut and serve banana stem curry with rice and mor kozhambu or rasam.