Begin by heating a tablespoon of oil in a large skillet over medium-high heat. Add the cubed beef in batches, searing until browned on all sides, which takes about 5-7 minutes. Remove the beef and set it aside on a plate.

In the same skillet, reduce the heat to medium and add the diced onions. Sauté until they turn translucent and fragrant, approximately 3-4 minutes. Stir in minced garlic, Italian seasoning, and black pepper, and cook until the garlic is aromatic.

Transfer the sautéed onions and garlic mixture into your slow cooker. Layer in sliced carrots, diced celery, pearl barley, tomato paste, and bay leaves. Stir gently to combine all the ingredients evenly.

Pour the beef broth over the combined ingredients in the slow cooker, ensuring that the barley is well-covered by the liquid. Stir gently.

Cover the slow cooker with its lid and set it to cook on low for 6-8 hours or on high for 3-4 hours. Check for doneness towards the end of the cooking time.

Once the cooking time is complete, stir the soup. Mix in freshly chopped parsley before serving to elevate flavor and aroma.