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.