Preheat oven to 180?C/350?F/gas mark 4. Make an incision along the mackerel. Crush the ingredients for the filling in a pestle and mortar and stuff them into the mackerel. Place on a hot griddle pan and sear for two minutes on each side. For the cous-cous: Add an equal volume of boiling water to the cous-cous. Add the butter and cover with cling film for 2 minutes. Fluff with a fork while adding the remaining ingredients. For the chermoula: Place the garlic, herbs and spices in a blender and slowly add the olive oil until you get the consistency of a sauce. Season to taste.