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.