Preheat the oven to 190C/170C Fan/Gas 5. Cut the heads off the mackerel by making a cut on one side, behind the gills, through to the back bone. Cut the other side in the same way. Cut through the bone and discard the head. Remove the fins using kitchen scissors. Rinse the fish under cold water, inside and out, and pat dry with kitchen paper.
Using a small sharp knife, make diagonal cuts in the flesh on both sides of the fish. In a bowl, mix the Moroccan-style seasoning with the lemon juice to make a paste. Set half aside and, using a teaspoon, carefully press the rest into the cuts in the mackerel.
Put the spiced fish in an ovenproof dish and bake in the oven for 25 minutes. Put the cherry tomatoes on a baking tray and cook on a different shelf for the last 10 minutes. Heat the oil in a pan and cook the onion until soft. Add the pepper and cook for another 2 minutes, then stir in the reserved seasoning.
Add the broad beans and stock, then heat until boiling. Cover and simmer for 5 minutes. Add the couscous and apricots, and heat until simmering. Remove from the heat, cover the pan, and set aside for 10 minutes. Fluff up the couscous mixture with a fork and stir in the parsley. Serve with the fish.