Simmer the potatoes for about 20 mins
until tender, then drain and mash with
most of the butter, plenty of seasoning
and a little milk. Season and set aside.
Meanwhile, cut the fish into 2cm cubes.
Bring the milk just to a simmer in a large
frying pan – when you see a few small
bubbles, add the fish, onion and bay
leaves, then cover and cook for 6-8 mins.
Lift the fish out onto a plate and strain
the milk into a jug.
Melt the butter in a pan, stir in the flour
and cook for 1 min over a medium heat.
Take off the heat, pour in a little 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, then simmer for
5 mins, stirring continuously, until it coats
the back of a spoon. Remove from the
heat, season with salt, pepper and
nutmeg, then mix in the parsley.
Heat oven to 200C/180C fan/gas 6.
Divide the fish between 2 ovenproof
dishes, about 1.3 litres each and strain
any juices into the sauce. Scatter over
the prawns, then add the eggs to both
dishes if eating straightaway, or to just
one dish if freezing the other. Pour the
sauce over both. Spoon half of the mash
over one dish so it covers everything,
making sure it reaches the sides. Rough
up the top with a fork and dot with
remaining butter. Bake for 30 mins until
the top is golden. Freeze the remaining
fish and mash (see tips, below).