Add the sliced mushrooms, chopped onion, diced carrots, diced parsnips, and diced potato to the slow cooker.
Stir in the rinsed pearl barley, dried thyme, bay leaf, salt, and black pepper.
Pour the vegetable broth over the ingredients and stir once to combine.
Cover and cook on low for 7 hours until the barley is tender and the root vegetables are soft.
Remove the bay leaf, stir the stew, and let it rest for 5 minutes before serving.