Stir in 3 1/2 C. water or broth. Bring to a boil over medium-high heat. Reduce heat to low; simmer, covered, about 1 hour or until barley and rice are tender.
Heat remaining 1 tablespoon oil in large skillet over medium-high heat. Add mushrooms, celery, carrot and parsley; cook and stir 5 to 10 minutes or until vegetables are tender.
Stir vegetable mixture into rice and barley mixture.