Place the pearl barley, sliced mushrooms, diced onion, diced carrots, diced celery, and minced garlic into the slow cooker.
Pour the vegetable broth over the ingredients and add the dried thyme, bay leaf, salt, and black pepper.
Stir everything together to combine evenly.
Cover and cook on low for 7 hours until the barley is tender and the vegetables have softened.
Remove the bay leaf, stir in the chopped parsley, and serve.