Preheat the oven to 190C/170C Fan/Gas 5. Grease a shallow ovenproof dish with the butter. Put the haddock fillets in another dish and sprinkle with 4tbsp water. Cover and cook in the oven for 30 minutes or until the fish flakes easily.
Meanwhile, put the spinach in a large nonstick frying pan with 2tbsp water and cook over a medium heat, moving it around the pan with a wooden spoon for 4-5 minutes, or until thawed. Drain in a sieve and transfer to the buttered dish. Drain any liquid from the fish, discard the skin and any bones and flake the flesh. Put the fish on top of the spinach.
Shell and slice the eggs and put on top of the fish in an even layer.
Warm the sauce and gently pour over the eggs. Sprinkle with the cheese and breadcrumbs. Bake for 10 minutes.