To make the Tzatziki: Combine all the ingredients in a bowl and set aside.
To prepare the couscous: Place the couscous in a bowl, add the lemon zest, juice, olive oil, garlic, seasoning and sliced spinach, mix well. Boil the stock or water, then add it to the bowl of couscous and stir it through. Cover with cling film and leave for 10 minutes then fluff it up with a fork.
To cook the fish: Meanwhile spread the harissa over the flesh side of the hake fillets. Season with a little salt.
Heat a little oil in a non-stick frying pan over a medium heat. Add the fish, skin side down and cook for 4 to 5 minutes until the skin is crisp. Turn and cook for another minute or two until the fish is cooked. This will depend on the thickness of the fillets.
To serve: Divide the couscous and fish between 4 plates and serve with tzatziki, lemon wedges and salad.