Melt the butter in a wide skillet over medium heat, stir in the flour and cook for 1 minute until lightly browned.

Whisk in the beef broth, Worcestershire, salt and pepper and simmer for 5 minutes until the gravy coats a spoon.

Lower the heat, add the sliced roast beef and warm it gently to 165°F without boiling.

Toast the bread until firm enough to hold the gravy but not deeply crisp.

Place one toast slice on each plate, pile warm beef on top and spoon gravy over the entire sandwich.