Heat olive oil in a large Dutch oven over medium-high heat. Add beef cubes in batches, searing on all sides until browned. Remove and set aside.
In the same pot, add the diced onion and cook for 5 minutes until translucent. Add garlic and cook for 1 more minute.
Stir in carrots and celery, then cook for another 3–4 minutes.
Add tomato paste and stir until vegetables are coated and fragrant.
Pour in beef broth, add bay leaf, thyme, and return the browned beef to the pot. Bring to a boil.
Reduce heat to low, cover, and simmer for 45 minutes.
Stir in rinsed barley and cook uncovered for another 45–60 minutes until beef and barley are tender.
Season with salt and pepper to taste. Discard bay leaf and garnish with chopped parsley before serving.