Heat the olive oil in a large pot over medium heat. Add the ground beef and cook until browned, breaking it up with a spoon as it cooks.
Add the diced onion, sliced carrots, and diced celery to the pot. Cook for 5 minutes until the vegetables start to soften.
Stir in the sliced mushrooms and minced garlic. Cook for another 3 minutes.
Add the pearl barley, beef broth, diced tomatoes, dried thyme, salt, and pepper. Stir everything together.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 45 minutes until the barley is tender.
Taste and adjust salt and pepper if needed before serving.