In a 3-qt. saucepan, saute mushrooms and onion in butter until tender. With a slotted spoon, transfer mushrooms and onion to a bowl; set aside.
Stir flour into pan drippings; cook over medium heat until lightly browned. Stir in water until smooth. Add barley; bring to a boil. Reduce heat; simmer, uncovered, stirring constantly, for 15 minutes or until barley is tender.
Add bouillon, milk, cheese, Worcestershire sauce, parsley and pepper; cook and stir until bouillon is dissolved and cheese is melted. Add the mushroom mixture; heat through.