Add the Meatballs: Place the frozen or pre-cooked meatballs into your crockpot.
Combine the Base: Add the diced tomatoes, tomato paste, and broth to the crockpot. Stir to combine.
Layer the Veggies: Add in the carrots, celery, onion, and zucchini, layering them over the meatballs.
Season the Soup: Sprinkle in the Italian seasoning, minced garlic, salt, and pepper. Stir gently to mix everything.
Cook the Soup: Cover and cook on low for 6–8 hours or on high for 3–4 hours, allowing the flavors to meld together.
Add Greens: About 15 minutes before serving, stir in fresh spinach or kale if desired.
Serve and Enjoy: Serve the soup hot, topped with grated Parmesan cheese. For a heartier meal, add cooked pasta or rice to the soup before serving.