Heat oil in a 4-quart Dutch oven over medium-high heat.
Add onion, carrot, and celery; cook 6 minutes, stirring frequently.
Add mushrooms; cook 2 minutes. Stir in barley, 3 cups broth, water, and next 3 ingredients; bring to a boil. Reduce heat, and simmer, uncovered, 20 minutes or until barley is tender.
Combine gravy mix, 1 cup broth, and sherry, stirring with a whisk. Stir gravy mixture into barley mixture; bring to a boil. Reduce heat, and simmer, uncovered, 10 minutes. Stir in pepper. Discard bay leaf.