Heat the olive oil in a large pot over medium heat. Add the onion, carrots, and celery and cook for 5 minutes until the vegetables begin to soften.
Stir in the garlic and mushrooms. Cook for another 6 to 7 minutes until the mushrooms release their liquid and start to brown.
Add the pearl barley, vegetable broth, salt, and black pepper. Bring the soup to a simmer, then reduce the heat to low, cover, and cook for 35 minutes.
Stir in the fresh thyme leaves and simmer uncovered for 5 more minutes. Taste and adjust salt if needed before serving.