Add the rinsed pearl barley, vegetable broth, frozen mushrooms, frozen diced onions, dried thyme, bay leaves, salt, and black pepper to the slow cooker.
Pour in the coconut milk and stir everything together until the ingredients are evenly combined.
Cover and cook on low for 7 hours, until the barley is tender and the stew has thickened.
Remove the bay leaves, give the stew a final stir, and serve hot.