Ask your fishmonger to gut the mackerel and clean them, leaving the heads on or off, according to your preference.
Slash each side of the fish diagonally in three places. Mix the mustard, coriander, garlic and seasoning, and spoon it into the slashes. Press the breadcrumbs into the surface.
Use two roasting bags, and place two fish in each, tightly closing the bag’s but cutting a slice in each to let the steam escape.