Place the couscous in a bowl and pour over the boiling stock. Cover and set aside for 5–6 minutes until just tender.
Combine the dressing ingredients in a small bowl or jug.
Fork the pepper, onions and parsley through the couscous with 2 tablespoons of the dressing. Spoon the couscous salad onto plates and scatter over the flaked mackerel. Serve with the dressing on the side, for drizzling on top.