Season the beef. Pat the beef dry with paper towels, then season it with 1 teaspoon salt and 1/2 teaspoon black pepper.
Sear the beef. Heat the olive oil in a large cast-iron skillet over medium-high heat. Working in batches, sear the beef for about 2–3 minutes per side, until deeply browned on several sides. Do not overcrowd the skillet. Transfer the browned beef to a large soup pot.
Cook the vegetables. Reduce the heat to medium. Add the onion, carrots, and celery to the same skillet and cook for about 5–6 minutes, stirring occasionally, until they begin to soften and pick up the browned flavor from the skillet.
Add the garlic and tomato paste. Add the minced garlic and tomato paste at the same time. Cook for about 1 minute, stirring constantly, until fragrant and the tomato paste darkens slightly and coats the vegetables.
Deglaze the skillet. Pour 1 cup of the beef broth into the skillet. Scrape the bottom well to loosen all the browned bits, then pour everything from the skillet into the soup pot with the beef.
Build the broth. Add the remaining 7 cups beef broth, Worcestershire sauce, dried thyme, and bay leaves to the soup pot. Stir well and bring to a gentle simmer.
Simmer the beef. Cover and cook over low heat for about 45–50 minutes, keeping the soup at a gentle simmer, until the beef is beginning to become tender.
Add the barley. Stir in the rinsed pearl barley, 1/2 teaspoon salt, and 1/3 teaspoon black pepper. Simmer partially covered for another 30–40 minutes, stirring occasionally, until the barley is tender with a pleasant chew and the beef is fork-tender.
Finish the soup. Remove the bay leaves. Stir in the fresh parsley, then taste and add additional salt and black pepper if needed. If the soup has become thicker than you like, stir in a little extra beef broth.
Serve. Ladle the beef barley soup into warm bowls and finish with extra parsley and freshly ground black pepper. Serve hot with crusty or homemade bread on the side.