01 - Heat olive oil in a large soup pot over medium-high heat. Add beef cubes and brown on all sides, about 5 minutes. Remove and set aside.
02 - In the same pot, add diced onions, sliced carrots, and sliced celery. Cook for 5 to 6 minutes until softened. Add minced garlic and cook for 1 additional minute.
03 - Return browned beef to the pot. Stir in mushrooms if using, diced tomatoes with juices, rinsed barley, beef broth, water, dried thyme, dried parsley, bay leaf, black pepper, and salt.
04 - Bring mixture to a boil, then reduce heat to low. Cover and simmer, stirring occasionally, for 1 hour and 15 minutes.
05 - Remove lid and continue simmering uncovered for 20 to 25 minutes, until beef and barley are tender and the soup slightly thickens.
06 - Discard bay leaf. Taste and adjust seasoning if needed. Serve hot, optionally garnished with fresh parsley.