Roughly peel (not completely) and chop the raw banana. Peel and chop the yam too in similar sized pieces. Wash well and set aside to drain.
In a chatti (earthern pot), add the chopped vegetables along with water upto its level, turmeric powder and salt. Bring to boil.
Cook till the vegetables are almost done.
Meanwhile, grind the coconut with the other ingredients under "for grinding" till it is nice and smooth. Add a little water as needed.
Keep the flame on simmer. Whisk the yogurt well along with pepper powder and add into the cooked vegetables. Give a good mix.
Add the coconut paste and mix well. Simmer for around 5-8 minutes till the raw smell is gone. Switch off and adjust seasoning.
To temper, heat coconut oil, splutter mustard seeds. Fry the dry chillies and curry leaves briefly and add into the gravy.
Close and allow the mixture to rest till the time of serving.