In a pan,add the chopped yam and plantain.Add green chilies, turmeric powder ,black pepper powder and sufficient quantity of water.Cover and cook until half done.Once it is half cooked add the salt ,ghee and curry leaves.Cook until soft and the gravy coats the pieces. Meanwhile add all the ingredients listed under 'for coconut paste' to form a smooth paste.(If you are not able to grind it ,add 1 or 2 tbsp of yogurt,just to run the blender.)Set aside until required.
Once the vegetables are done,add the yogurt( whisk to remove lumps) and cook until it turns thick.Make sure you stir the curry well,in between to avoid burning.
Now,add the coconut paste , salt to taste and cook over medium flame for about 10 minutes or until the mixture comes to a boil.Switch off the flame.(If the curry is sour , add 1 or 2 tsp jaggery to balance the taste.)
For tempering ,heat oil in a pan,add the mustard seeds and let it splutter.Add the curry leaves and switch off the stove.Add the fenugreek powder and immediately add this to the kalan and stir gently.Cover and set aside for about 10 minutes for the flavours to mingle.Serve with rice.