Whole fish with mayonnaise sauce

Preheat the oven to 180 ºC (350 ºF).
Grease a large sheet of aluminium foil with butter.
Make three diagonal slashes across both sides of the fish. Season the fish well inside and out with salt and pepper and place on the greased side of the aluminium foil. Place on a baking sheet.
Mix the mayonnaise, onion, green pepper and tomato and spoon into the cavity of the fish. Spoon some of the mixture over the fish as well, wrap it in the foil and bake for about 30 minutes or until done. Open the foil a few minutes before the end of baking time to allow the fish to brown lightly. The fish is done when it flakes easily with a fork.
Serve with the remaining mayonnaise sauce, lemon wedges and a salad if desired.