Wash the mackerel, dry with kitchen paper, season with salt and pepper and place in a baking dish.
Sprinkle with diced tomatoes, wafer-thin slices of garlic, coarsely chopped parsley and - for those who prefer it a little spicier - chopped chilies. Douse lightly with olive oil and cook in the preheated oven for about 30 minutes.
The fish is done when the bones can be easily removed from the meat. Serve with bread to soak up the sauce.