Combine first 4 ingredients in med. saucepan; bring to boil. Add barley; cover, reduce heat and simmer 10 min. Remove from heat, let stand 5 min.
While barley stands, coat large non-stick skillet with cooking spray and place over med-high heat until hot. Add mushrooms, onion and celery; cook 3 min. or until tender, stirring often. Stir into cooked barley.
