Cook the vegetables:
Add diced banana and yam to a pan.
Add turmeric powder, pepper powder, salt, and just enough water to cover.
Cook until soft but not mushy.
Grind coconut mixture:
Grind grated coconut, green chilies, and cumin seeds to a smooth paste with a little water.
Add coconut paste:
Mix the ground coconut paste into the cooked vegetables.
Cook for a few minutes till the raw smell disappears.
Add curd:
Lower the flame completely.
Add well-beaten sour curd gradually, stirring continuously.
Simmer gently until it thickens. (Do not allow it to boil after adding curd.)
Tempering:
Heat coconut oil, splutter mustard seeds, add fenugreek seeds, broken red chilies, and curry leaves.
Pour this tadka over the Kaalan.
Mix well and serve!
Tastes even better after a few hours as the flavors settle.