Melt butter in large saucepan over medium heat. Stir in flour until smooth; cook 1 minute, stirring constantly. Stir in milk, beer, Worcestershire sauce, mustard, salt and pepper. Heat to a boil, stirring frequently. Reduce heat; simmer 10 minutes, stirring frequently.
Stir in cheese until melted. Ladle soup into bowls. If desired, serve with croutons.