Heat a large pot over medium heat and add the ground beef. Cook until browned, breaking it up with a spoon as it cooks.
Add the onion, carrots, celery, and garlic to the pot. Stir and cook for 5 minutes until the vegetables begin to soften.
Stir in the pearl barley, diced tomatoes, 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 35 minutes, stirring occasionally, until the barley is tender.
Remove the bay leaf and stir in the fresh parsley just before serving.