Preheat the oven to 200°C and grease a large piece of foil with butter.
Slash each side of the fish three times and insert chopped coriander into the gaps.
Place coriander stalks in the fish cavity and season with salt and pepper.
Wrap the fish in foil to create a parcel with the seam on the side.
Bake for 30 minutes, turning the parcel over after 15 minutes.
Mix chopped cucumber, chilli, tomatoes, grapefruit, lime, ginger, oil, and chutney together.
Transfer the cooked fish to a platter and remove the skin.
Gently heat the salsa in a pan, stir in the final coriander, and serve over the fish.