Place the cheese, butter, Worcestershire sauce, mustard, flour and beer into a small saucepan.
Heat gently, stirring until the cheese melts and all the ingredients blend together to form a thick paste. Allow to cool slightly.
Meanwhile toast the bread under a grill on one side. Take out of the grill and turn over.
Spread the cheese mixture over the bread and return to the grill. Cook until the cheese bubbles and turns golden brown.