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.