01 - Heat olive oil in a large soup pot over medium-high heat. Add beef cubes and brown on all sides for about 5 minutes. Remove meat and set aside.
02 - In the same pot, add onions, carrots, and celery. Cook for 5 to 6 minutes until softened. Stir in minced garlic and cook for 1 minute.
03 - Return beef to the pot, add mushrooms if using, diced tomatoes with juice, pearl barley, beef broth, water, dried thyme, dried parsley, bay leaf, black pepper, and salt.
04 - Bring mixture to a boil, reduce heat to low, cover, and simmer for 1 hour 15 minutes, stirring occasionally.
05 - Remove lid and continue to simmer uncovered for 20 to 25 minutes until beef and barley are tender and soup has thickened slightly.
06 - Remove bay leaf, adjust seasoning as needed, and serve hot. Garnish with fresh parsley if desired.