Prep the veggies. Wash and shred the cabbage. Chop carrots, celery, onion, and mince the garlic.
Optional sauté. In a skillet, heat olive oil and sauté onion and garlic until fragrant, about 3 minutes. This step adds extra flavor but can be skipped if you're short on time.
Layer into crockpot. Add cabbage, carrots, celery, onion, garlic, and diced tomatoes to the slow cooker.
Pour in broth. Add vegetable broth and sprinkle in salt, pepper, and Italian seasoning.
Cook. Cover and cook on low for 6–8 hours or high for 3–4 hours, until the vegetables are soft and flavors have melded.
Taste and adjust. Add more salt or seasoning if needed before serving.
Serve. Ladle into bowls and serve warm. Optional: garnish with parsley or a sprinkle of parmesan.