In the base of your slow cooker, combine the diced tomatoes and their juice, tomato paste, beef broth, soy sauce, stirring until well mixed. Add the thyme, bay leaves, onion, garlic, carrots, celery.
Season the beef all over with salt and pepper; add to the slow cooker, pushing it down to cover with the liquid. Cover and cook on low until the beef is so tender that it can easily be pulled apart with a fork, 9-10 hours (or cook on high for 6-7 hours)
Transfer the beef to a large plate or cutting board and use two forks to shred into bite-size chunks, discarding any pieces of fat and grisle. Discard the bay leaves.
Stir the pearl couscous into the soup in the slow cooker. Cover and cook until tender, 10-15 minutes. Return the shredded beef to the soup. Add the parsley. Taste and season with more salt and pepper if desired. Serve.