Add the beef stew meat to the bottom of the slow cooker.
Layer the chopped onion, sliced carrots, sliced celery, and minced garlic on top of the meat.
Pour in the rinsed barley, diced tomatoes with their juice, and beef broth.
Sprinkle in the dried thyme and add the bay leaf. Season with salt and pepper.
Stir the ingredients gently to combine everything.
Cover and cook on low for 8 hours or on high for 4 to 5 hours until the beef is tender and the barley is soft.
Remove the bay leaf before serving.