Marinate fish pieces with a mixture of chilly powder, turmeric powder, pepper powder and ginger garlic paste. Keep aside for 15 minutes.
Fry the fish pieces to a slight golden color and keep aside.
Mix coconut milk powder in 3 cups of lukewarm water.
Heat oil in a pan and splutter mustard seeds. Add onion, garlic, green chillies and ginger. Saute for sometime.
When onion turns transparent, add coriander, pepper, turmeric and garam masala. Saute well.
Mix well and add diced tomatoes. Fry with very little water till raw smell goes.
Add coconut milk along with curry leaves and vinegar. Add required amount of salt and allow to boil.
When boiling starts, add semi fried fish. Keep on low flame till fish gets completely cooked.
Before removing from flame, add finely chopped coriander leaves.
Allow for one quick boil and off flame. Leave for 15 minutes and serve.