Place all the sauce ingredients in a stainless steel saucepan and heat gently over a low heat for 3-5 mins, until combined and reduced to a coating consistency. Remove from the heat, discard the ginger and allow to cool.
Preheat the grill to a high heat.
Pat the mackerel dry with kitchen paper. Make 2 or 3 deep slashes in the side of each fish and lightly season the cavities; add 2 half slices of lemon to each fish.
Place the mackerel in a shallow roasting tin or edged baking sheet. Brush both sides with melted butter and season lightly.
Reduce the grill to a medium heat and cook the fish placed 5-7.5cm (2-3in) from the heat source for 6-8 mins, then turn over and grill the other side. Serve sprinkled with the chopped parsley and with the warm rhubarb sauce.
If cooking the mackerel on a barbecue, use a fish rack to make turning the fish easier.
Omit the lemon and use slices of fresh orange.
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