Place the whole peppers under a hot grill and turn them frequently until the skin turns black. Once blackened, place peppers in a plastic food bag and secure the top. The steam from the peppers will make it easier to remove the skin.
Bake the garlic bread slices according to packet instructions.
After 5-10 minutes, remove the peppers from the bags and carefully start to remove the skin. This may be easiest under gentle running water. Remove the stalk and seeds then cut the pepper into long thin strips.
Remove the skin from the mackerel fillet and the centre bone strip. Flake the mackerel into a bowl and fold in the peppers.
Top each piece of garlic bread with the pepper and flakes of mackerel.
Just before serving, drizzle with olive oil, season with salt and black pepper and top with Parmesan shavings.
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