Pre-heat the oven to 220°C/Gas Mark 7. Place the spinach in microwavable dish with a little water and microwave for 1 minute to wilt. When cooled slightly, roughly chop. Mix the chives with the cheese sauce.
Add 2 tablespoons of the sauce to the spinach, season with black pepper and nutmeg, then arrange in a large ovenproof dish. Place the fish fillets and chopped eggs on top of the spinach and season again.
Scatter the mozzarella over the surface, then spoon the rest of the sauce evenly over the fish, using a spatula to spread it to the corners. Mix the Parmesan with the breadcrumbs and scatter this over the sauce. Place the dish in the oven and bake for 25-30 minutes or until the top is golden. Serve with peas.