Cut fish fillets into 5cm pieces and set aside.
Heat a tablespoon of oil in a large, wide pan with a lid, over a medium heat. Add the onion and garlic and cook for 10 minutes or until the onion and garlic soften. Add the cumin, paprika, ground coriander and turmeric, stir well and cook for 1 minute. Add in the cinnamon stick, tomatoes, courgette and water and cook for 10 minutes. Then add the lemon zest, reserve the juice for the couscous, and half the chopped coriander. Taste and season with salt and pepper. Season the fish and then place fish in a single layer over mixture in pan. Cover and cook for 5 minutes.
Meanwhile, place the couscous in a heatproof bowl, add the lemon juice, the remaining tablespoon oil and 250ml boiling water. Stir well, cover and set aside for 3-4 minutes or until the liquid is absorbed.
Use a fork to separate the grains. Taste and season with salt and pepper. And stir through some of the remaining chopped coriander. Keep some to sprinkle over the tagine just before serving.
To serve: Spoon the couscous into serving bowls and top with the fish tagine. Sprinkle with the remaining coriander.