Place the fish in a pan and pour over
the milk. Bring to a simmer and gently
poach for 2 mins until opaque and just
cooked. Lift out the fish, remove the skin
and flake. Reserve the milk.
Heat grill to high. Melt the butter in a
small pan, then stir in the flour for 1 min.
Gradually whisk in the reserved milk,
followed by the ale or beer. Cook, whisking
all the time, until the sauce is thickened
and smooth. Stir in the mustard and most
of the cheese and season with pepper.
Top the bread with the fish, spoon over
the sauce and scatter with the remaining
cheese. Grill until golden and bubbling.