Preheat the oven to 200C/180C Fan/Gas 6. Grease a large piece of foil with butter.
Using a sharp knife make three slashes along each side of the fish, and push a little chopped coriander into the slashes. Place the coriander stalks into the cavity of the fish. Season all over with salt and pepper. Wrap the fish in foil to make a parcel. Make sure the seam of the parcel is to the side, so you can turn the fish upside down. Bake for about 30 minutes, turning over after 15 minutes, until cooked through.
To make the salsa, chop all the ingredients. Put them all into a bowl, apart from the coriander, and mix together. Season with salt and pepper.
Remove the foil, serve the sea bass on a large platter and carefully remove the skin. Heat the salsa gently in a pan, add the coriander and pour over the top or serve alongside.