1) Preheat the oven to 180 degrees Gas 5 2) Pour boiling water over the spinach in a colander to wilt. 3) Meanwhile, run your fingertips over the surface of the skinned fish and remove any stray bones with tweezers, then pat the fish dry with kitchen paper. 4) Return to the spinach and squeeze it in your hands very tightly to get rid of every last drop of water – you need to get it as dry as possible. 5) Then place it on a clean board and chop it finely. 6) To make the cheese sauce melt the butter in a saucepan and once melted add the flour and mix to a thick consistency then add the milk. 7) Use a whisk to remove any lumps and then add the cheddar cheese, stir until the cheddar has melted. 8) Add 2 tablespoons of the sauce to the spinach and mix together, season it with salt, black pepper and nutmeg, then arrange it in the dish. 9) Next, place the fish on top of the spinach and season again, tear and scatter the mozzarella over the fish, then spoon the rest of the cheese sauce evenly over the fish, using a spatula to spread it to the corners. 10) Now mix the Parmesan with the parsley and scatter this over the sauce. 11) Bake in the oven for 40 minutes, or until the top is golden.