Simple but tasty - bake them with home-made chips on shelf above. I just place the whole, gutted mackerel in an oiled dish (can stuff them with herbs, e.g. dill) place on centre shelf (Mk. 5 for 30 - 35 mins) with - 2 shelves up from this - chips (rolled in oil, maybe some paprika, and salt) on an oiled baking sheet.
Should both be done at the same time, depending on how thick the chips are and whether there are too many!
You could go crazy and replace the chips (same shelf) with sliced courgettes and red peppers, oiled - roast veg.