Season the beef with salt and pepper. Heat a skillet over medium heat and brown the pieces on all sides. Transfer the beef to the slow cooker.
Add the onion, carrots, celery, and garlic to the slow cooker. Stir in the tomato paste and thyme until the vegetables are coated.
Pour in the beef broth and add the barley and bay leaf. Stir once to combine.
Cover and cook on low for 7 to 8 hours, until the beef is tender and the barley is cooked through.
Remove the bay leaf. Taste and adjust salt and pepper if needed before serving.