Heat oil in anon-stick pan. Add mustard seeds and let them splutter.
Add curry leaves, onion, ginger and green chillies. Mix well and saute till onion is soft.Add turmeric powder, salt and thin coconut milk. Stir to mix and bring it to a boil.
Add fish pieces and mix well. Cook till the fish is soft. Reduce the heat and add thick coconut milk and fenugreek powder.Mix well and remove from heat. Serve hot.