Preheat oven to 350°.
Bring beef broth and barley to a boil in a small saucepan. Cover, reduce heat, and simmer 10 minutes or until the liquid is absorbed.
Cut each bell pepper in half lengthwise, and discard seeds and membranes. Arrange bell pepper halves in a 9-inch pie plate. Cover with heavy-duty plastic wrap. Microwave at HIGH 5 minutes or until crisp-tender; drain.
Cook beef, mushrooms, and garlic in a nonstick skillet over medium-high heat 4 minutes or until browned, stirring to crumble. Add barley, 2 tablespoons green onions, tomato puree, and next 5 ingredients (tomato puree through black pepper), and cook 2 minutes or until thoroughly heated. Divide beef mixture evenly among pepper halves. Top each pepper half with 2 tablespoons cheese and 3/4 teaspoon onions. Bake at 350° for 10 minutes or until cheese melts.