Melt butter in a medium saucepan over medium heat. Whisk in flour and cook for 1 to 2 minutes while stirring constantly to make a roux. Add beer and bring to slow boil. Reduce heat and simmer, stirring occasionally until mixture thickens to the consistency of heavy cream. Add Worcestershire, mustard and cayenne, stirring thoroughly. Add cheese 1 cup at a time, melting cheese after each addition. Stir fondue until it is smooth. Turn into fondue pot and serve.
From Food Network