Sauté the sliced onions, slit green chilies, ginger paste, and turmeric powder in a pan until fragrant.

Add the thin coconut milk, salt to taste, and fish pieces. Cook gently until the fish is partially cooked.

Stir in the vinegar and continue to cook until the fish is fully done and the gravy has thickened slightly.

Finally, add the thick coconut milk and heat through without boiling. Check seasoning and remove from heat immediately.