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Poach 400g skinless white fish fillets and 400g skinless smoked haddock fillets. Put the fish in the frying pan and pour over 500ml of the full-fat milk. Quarter 1 small onion and stud each quarter with a clove, then add to the milk, with 2 bay leaves. Bring the milk just to the boil – you will see a few small bubbles. Reduce the heat and simmer for 8 mins. Lift the fish onto a plate and strain the milk into a jug to cool. Flake the fish into large pieces in the baking dish.
Hard-boil 4 eggs. Bring a small pan of water to a gentle boil, then carefully lower the eggs in with a slotted spoon. Bring the water back to a gentle boil, with just a couple of bubbles rising to the surface. Set the timer for 8 mins, cook, then drain and cool in a bowl of cold water. Peel, slice into quarters and arrange on top of the fish, then scatter over the chopped leaves of a small bunch of parsley.
Make the sauce. Melt 50g butter in a pan, stir in 50g plain flour and cook for 1 min over moderate heat. Take off the heat, pour in a little of the cold poaching milk, then stir until blended. Continue to add the milk gradually, mixing well until you have a smooth sauce. Return to the heat, bring to the boil and cook for 5 mins, stirring continually, until it coats the back of a spoon. Remove from the heat, season with salt, pepper and a pinch of freshly grated nutmeg, then pour over the fish.
Assemble and bake. Heat oven to 200C/fan 180C/gas 6. Boil 1kg floury potatoes, cut into even-sized chunks, for 20 mins. Drain, season and mash with the remaining 50g butter and 100ml full-fat milk. Use to top the pie, starting at the edge of the dish and working your way in – push the mash right to the edges to seal. Fluff the top with a fork, sprinkle over 50g grated cheddar, then bake for 30 mins. Make up to a day ahead, chill, then bake for 40 mins.