Cook the potatoes in boiling salted
water for 5-7 mins, until almost tender
but firm enough to grate. Drain and
refresh under cold running water. Put
the coley in a shallow saucepan and
pour over the milk. Bring to a simmer
and cook for 5 mins. Drain the fish,
reserving the milk, then set aside.
Heat half the butter in a medium
saucepan, add the leek, then cook
for 5-6 mins until softened. Stir in the
flour for 1 min, then remove from
the heat. Gradually add the milk, stirring
well between each addition. Return to
the heat and stir until the sauce comes
to the boil. Simmer for 2 mins, then
stir in the parsley and mustard.
Heat the grill to high. Flake the fish
into large chunks, fold into the sauce,
then place in a small ovenproof dish.
Coarsely grate the potatoes. Melt the
remaining butter, toss with the potatoes,
season and scatter over the dish. Place
under a medium grill for 5-10 mins until
the potatoes are golden and tender.