Put the milk into a saucepan with the
onion, peppercorns, mace, bay leaf and parsley stalks. Bring to the
boil, then remove from the heat and leave to infuse while preparing the
remaining ingredients. Melt the butter in another saucepan and use a
little to brush the inside of a 1.6 liter / 2.75 pint ovenproof pie
dish. Cut the cod into finger-size strips, place in the pie dish and
scatter the mushrooms and prawns on top.
Strain the milk through a sieve, then
discard the contents of the sieve. Add the flour to the remaining butter
in the saucepan, stirring to blend. Gradually stir in the strained milk
to make a smooth sauce. Bring to the boil, stirring constantly until the
sauce thickens. Season with salt, pepper and lemon juice and add
two-thirds of the grated cheese. Stir until melted.
Pour the sauce over the fish, sprinkle
with the remaining cheese, place on a baking sheet and cook in a
preheated oven, 180C (350F), Gas Mark 4, for 20-25 minutes until golden
brown. Serve with country bread and salad.