Gently warm the apple cider vinegar and add the saffron. Let it infuse for 5 minutes.
Combine all the ingredients including the saffron and vinegar in a mortar and pound till you have a rough paste or process in a food processor. The chermoula should be coarsely ground.
Transfer to an airtight jar and store in the refrigerator.
To make the grilled mackerel
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.