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 soften.
Stir in the garlic and mushrooms. Cook for another 4 minutes until the mushrooms release their liquid and begin to brown.
Add the rinsed barley, vegetable broth, thyme sprigs, salt, and black pepper. Stir to combine.
Bring the soup to a boil, then reduce the heat to low. Cover and simmer for 35 minutes, stirring occasionally, until the barley is tender.
Remove the thyme sprigs before serving. Taste and adjust salt if needed.