Heat the olive oil in a large pot over medium heat. Add the onion and cook for 3 minutes until it starts to soften.
Stir in the garlic and mushrooms. Cook for 5 minutes, stirring occasionally, until the mushrooms release their liquid and begin to brown.
Add the carrots and celery. Cook for another 3 minutes to soften the vegetables slightly.
Pour in the vegetable broth, then add the rinsed barley, thyme, bay leaf, salt, and black pepper. Stir to combine.
Bring the mixture to a boil, then reduce the heat to low. Cover and simmer for 35 minutes, stirring once or twice, until the barley is tender.
Remove the bay leaf. Stir in the chopped parsley just before serving.