Heat a large pot over medium heat and add the onion, carrots, and celery. Cook for 5 minutes until the vegetables start to soften.
Stir in the garlic and cook for 1 minute more.
Add the pearl barley, beef broth, diced tomatoes, thyme, salt, and pepper. Bring the mixture to a boil.
Reduce the heat to low, cover the pot, and simmer for 30 minutes until the barley is tender.
Stir in the cubed leftover meatloaf and the green beans. Simmer uncovered for 10 minutes until the meatloaf is heated through and the green beans are crisp-tender.
Taste and adjust salt or pepper if needed before serving.