Preheat the oven to 200°C/180°C/gas mark 6. Brush a piece of foil with oil.
Place the cod fillets on top of the foil, side by side, and spread over the harissa paste. Roll up the foil into a parcel and bake in the oven for 15 minutes.
While the fish is cooking, place the couscous into a heatproof bowl, pour over the boiling stock, stir and cover. Leave to stand for 10 minutes, then fluff up with a fork. Add the spring onions, pepper, olive oil and lemon juice and season with salt and black pepper.
Divide the couscous between 2 plates and top with the fish, with any cooking juices drizzled over. Garnish with the coriander, if using.