Add the sliced mushrooms, rinsed barley, diced onion, sliced carrots, sliced celery, and minced garlic to the slow cooker.
Pour in the vegetable broth and add the thyme leaves, bay leaf, salt, and black pepper.
Stir once to combine everything evenly.
Cover and cook on low for 7 hours.
Remove the bay leaf, taste, and adjust salt or pepper if needed before serving.