1) Heat large pot over medium-high heat. Coat with cooking spray. Add mushrooms and onion; sauté 10 minutes or until lightly browned. Spoon onion mixture into a medium bowl. Recoat pot with cooking spray. Add carrots, celery, parsnip and minced garlic; sauté 4 minutes or until lightly browned. Add carrot mixture to onion mixture in a bowl.
2) Heat oil in pot over medium-high heat. Add beef; cook 3 minutes, browning on all sides. Add 1 cup broth to pan, scraping pan to loosen browned bits. Add remaining 5 cups broth, onion mixture, 2 cups water, salt, pepper and thyme. Bring to a boil; cover and reduce heat to medium-low, and simmer 1 hour or until beef is tender.
3) Discard thyme sprigs (if used). Stir in barley; cover and cook 30 minutes or until barley is al dente. Cook an additional 15 minutes uncovered.