In a saucepan, melt butter; stir in flour, salt if desired, pepper, mustard and Worcestershire sauce until smooth. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened. Reduce heat. Add the cheese; cook and stir until melted. Transfer to a fondue pot or slow cooker, keep warm. Serve with bread or ham. Yield: 2-1/2 cups.