Brown the ground beef in a skillet over medium heat, breaking it up as it cooks, then drain excess fat.
Add the browned beef to the slow cooker along with the rinsed barley, carrots, potatoes, parsnip, onion, and garlic.
Pour in the beef broth and stir in the thyme, salt, and pepper.
Cover and cook on low for 7 to 8 hours until the barley is tender and the vegetables are soft.
Stir once halfway through if possible, then taste and adjust seasoning before serving.