Heat a large skillet over medium heat and add the ground beef. Cook until browned, breaking it up with a spoon as it cooks. Drain excess fat if needed.
Add the diced onion and minced garlic to the skillet with the beef. Cook for 2 minutes until the onion starts to soften.
Transfer the beef mixture to the slow cooker. Add the rinsed barley, sliced carrots, sliced celery, beef broth, diced tomatoes, dried thyme, and bay leaf.
Stir everything together, then season with salt and pepper. Cover and cook on low for 7 to 8 hours or on high for 4 hours until the barley is tender.
Remove the bay leaf before serving. Taste and adjust salt and pepper if needed.