Hi Jenny. You are another lucky one in having lots of mackrel, it is one of my favourite fish. Mackrel has a very high content of natural oil so I would omit any butter for cooking them. Usually they are tasty enough plain grilled or barberqued served with wedges of lemon. Or after cleaned and deboned open them flat sprinkle them with salt and pepper add some fennel leaves (bronze or green), roll them and arrange in a terrine tail up. Between them put small bayleaves and crushed juniper berries. Sprinkle the lot with fresh breadcrumps, cover with tinfoil and bake till cooked. When you lift the tinfoil you will know what life is all about and please do not forget the wine.
Regards
Don Vincenzo