Put the couscous in a bowl with the harissa paste, half the crushed cumin seeds, the red onion and the raisins. Pour over 325ml boiling water, then cover with clingfilm and leave for 10 minutes. Fluff up with a fork and season with freshly ground black pepper.
Preheat the oven to 180°C/gas mark 4. Cut 4 x 50cm square pieces of baking parchment.
Stir the courgette, carrot and half the parsley into the couscous and combine thoroughly. Divide the couscous among the parchment squares, placing it in the middle, then place a cod fillet on top of each. Top with some slices of lemon and a little of the remaining parsley. Fold up the paper squares to make sealed parcels. Place on 2 baking trays and cook in the oven for 15-20 minutes.
Meanwhile, in a small bowl, mix the yogurt with the remaining crushed cumin seeds, the garlic and the remaining parsley. Serve the fish parcels with the yogurt and lemon wedges on the side.