Combine pearled barley, vegetable broth, and diced tomatoes in the slow cooker. Stir until grains are evenly coated.

Layer diced carrots, celery, bell peppers, and zucchini over the base.

Sprinkle minced garlic, dried thyme, bay leaf, salt, and pepper over the vegetables.

Cover and cook on low for 6–8 hours (or high for 3–4 hours) until barley is tender and vegetables are soft.

Remove the bay leaf, stir gently to blend flavors, then taste. Adjust salt and pepper as needed.

Ladle into bowls and top with chopped fresh parsley and red pepper flakes.