Heat a large pot over medium heat and add the ground beef. Cook until browned, breaking it apart with a spoon. Drain any excess fat.
Add the onion, carrots, and celery to the pot. Cook for 5 minutes until the vegetables begin to soften.
Stir in the garlic and cook for 1 minute until fragrant.
Add the pearl barley, diced tomatoes with their juice, beef broth, thyme, and bay leaf. Season with salt and pepper.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 45 to 60 minutes, stirring occasionally, until the barley is tender.
Remove the bay leaf before serving.