In a large bowl toss cheese, cornstarch and mustard to coat cheese. In a 2 quart saucepan over low heat, heat beer and Worcestershire until hot, but not boiling. With a fork or wire whisk, gradually stir in cheese. Cook over low heat until cheese is melted and mixture is smooth and bubbling, stirring constantly. Pour into fondue pot for serving. Keep warm over low heat on fondue pot.