Place the haddock in a single layer in a deep frying pan with a lid and pour over enough boiling water to cover. Bring to the boil, cover and simmer gently for 2 minutes, until half cooked. Carefully transfer the fish fillets to a shallow ovenproof dish and keep warm.
Meanwhile, place the cheese sauce in a small pan and heat through. Remove from the heat and stir in the mustard, egg yolks, Worcestershire sauce and parsley.
Top the fish with the mixture, sprinkle over the grated cheese and cook under a preheated hot grill for 3-4 minutes until bubbling and golden. Serve at once with mashed potatoes and peas.