Wash and clean fish. Make a slit at one side. Mix all the chopped ingredients together: onion, green chillies, tomato, ginger, vinegar, curry leaves with required salt. Stuff the mixture inside the fish.
Place the fish on a piece of silver foil. Drizzle a little coconut oil over it. Sprinkle salt and cover with another piece of foil. Fold the edges so that the gravy doesn't leak out.
Place the fish on a tray and bake for 10-15 minutes at oven temperature of 180 degrees.
When done, you can sprinkle a little more coconut oil before serving with rice, else add chopped coriander leaves and relish with bread.