Heat a large pot over medium heat and add the beef stew meat. Cook until browned on all sides, stirring occasionally.
Add the onion, carrots, celery, and garlic to the pot. Cook for 5 minutes until the vegetables begin to soften.
Stir in the pearl barley, diced tomatoes, beef broth, thyme, salt, pepper, and water.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 45 minutes, stirring once or twice.
Check that the barley is tender and the beef is cooked through. Remove from heat and let cool slightly before portioning.