Remove the outer covering or thick hard shells covering the tender plantain stem and discard them.
Cut the plantain stem into roundels and further chop them fine. Remove any fibre that comes off while chopping and discard them.
You can soak the chopped plantain stem in a mixture of buttermilk and water to avoid any discoloration. This step is purely optional.
Soak the tur dal in enough water for 15 minutes or before you begin chopping the stem.
Heat a pan with oil. Add mustard seeds and when they crackle, add urad dal and channa dal and once they become golden brown, add broken dry chilies, asafetida and sauté for a few seconds.
Now add the soaked tur dal after draining the water away, chopped plantain stem, salt and mix well. Close with a lid and cook for 2 minutes.
Now add sambar powder, turmeric powder, mix well and let it cook over low flame. Keep mixing in between for even cooking of the plantain stem. No need to close with a lid or add water. The moisture in the stem is enough to cook. It should take 5-7 minutes to get cooked till the stem becomes soft.
Before taking off add grated coconut, curry leaves, mix well and take off fire. Close with a lid and leave it for 2-3 minutes for the flavors to merge together.
Vazhai thandu poriyal is ready.
Serve as a side dish with hot steaming rice.