Preheat your grill to maximum.
In a saucepan add the cheese, butter, mustard, Worcestershire sauce and flour and combine. Add the beer and season with salt and pepper.
Cook over a low to medium heat until the cheese has melted and you're left with a delicious thick sauce.
Place the bread under the grill and toast on one side.
Pour the cheese sauce over the untoasted side of bread and grill until golden.