Place the lamb stew meat, diced onion, and minced garlic into the slow cooker.
Add the sliced carrots, sliced parsnips, diced potatoes, and pearl barley.
Pour in the beef broth and balsamic vinegar.
Sprinkle the dried rosemary, dried thyme, salt, and black pepper over the top.
Stir everything gently to combine the ingredients evenly.
Cover and cook on low for 8 hours until the lamb is fork-tender and the barley is soft.
Stir once more before serving to distribute the vegetables and barley.