Coat the mackerel with the chermoula making sure to rub some of the marinade into the slits on the sides of the fish. Marinate for 1 hour.
Preheat the broiler in the oven to 180°C.
Heat ½ tbsp olive oil in a pan on medium heat and lightly sear the fish, cooking for no more than 30 seconds on each side.
Place the fish on a baking sheet and grill in the oven for 15 minutes. Serve with additional chermoula on the side.