1. Heat the olive oil in a large pot over medium heat.
2. Add the diced onion and cook until translucent, about 5 minutes.
3. Stir in the minced garlic and cook for another minute.
4. Add the sliced mushrooms and cook until they release their juices, about 7-10 minutes.
5. Pour in the vegetable broth and bring to a boil.
6. Stir in the pearl barley, dried thyme, salt, and black pepper.
7. Reduce the heat to low, cover, and simmer for 45 minutes, or until the barley is tender.
8. Adjust seasoning to taste and serve hot.