Place the beef cubes in the slow cooker and sprinkle the flour, salt, and pepper over them. Toss to coat evenly so the flour will thicken the broth into gravy as it cooks.
Add the diced onion, minced garlic, sliced carrots, shredded cabbage, pearl barley, dried thyme, and bay leaf to the slow cooker.
Pour the beef broth over the ingredients and stir gently to combine everything and distribute the flour.
Cover and cook on low for 8 hours until the beef is fork-tender and the barley is plump.
Remove the bay leaf, taste the stew, and adjust salt and pepper if needed before serving.