Cook barley in a large saucepan over medium heat 5 minutes or until golden, stirring occasionally. Add water; bring to a boil. Cover, reduce heat, and simmer 50 minutes or until barley is tender and liquid is absorbed. Place barley in a large bowl; cool slightly.
While barley cooks, heat olive oil in a large nonstick skillet over medium heat. Add mushrooms, celery, and shallots; cook 5 minutes or until mushrooms are tender and liquid evaporates, stirring frequently. Remove from heat; stir in chopped parsley and rosemary. Cool slightly. Add mushroom mixture, Simple Roasted Leg of Lamb, goat cheese, balsamic vinegar, salt, and pepper to barley. Cover and chill.