Meanwhile, in a small saucepan, saute onion in butter until tender. Stir in flour until blended. Gradually stir in milk. Bring to a boil over medium heat, stirring constantly; cook and stir for 2 minutes or until slightly thickened. Reduce heat to low. Stir in 1/2 cup cheese, mustard, Worcestershire sauce, pepper and salt until cheese is melted. Stir in broccoli. Cover; keep warm.