Toast the bread on both sides under broiler or toaster.
To a saucepan, add the butter over medium heat. When the butter is melted and sizzling, whisk in the flour and cook for a minute, stirring.
Slowly whisk in the ale until there are no lumps. Whisk in the Worcestershire sauce and mustard powder. Use a spatula and stir in the cheese until it is completely melted and the sauce is smooth.
Remove from the heat and transfer to a bowl to slightly cool and thicken.
Preheat broiler.
Spread the cheese sauce on all of the slices of bread. Broil until bubbly, browned and melted. Serve immediately