Add zucchini, cauliflower, onion, garlic, celery, and carrots (if using) to the slow cooker.
Pour in diced tomatoes and tomato paste.
Add vegetable broth and stir to combine all ingredients.
Add Italian seasoning, salt, and pepper.
Cover the Crockpot and cook on low for 6-8 hours or high for 3-4 hours, until vegetables are tender.
About 30 minutes before serving, add chopped spinach or kale.
Taste and adjust seasoning as desired.
Garnish with shredded parmesan cheese and fresh basil or parsley if desired.