To the slow cooker, add beef stock, zucchini, celery, onion, carrot, tomato, and garlic salt. Cover.
In a large mixing bowl, combine ground beef, Parmesan, garlic, egg, parsley, sea salt, onion powder, oregano, Italian seasoning, and pepper. Mix until all ingredients are well incorporated. Form into approximately 30 meatballs.
Heat olive oil in a large skillet over medium-high heat. Once the pan is hot, add meatballs and brown on all sides. No need to worry about cooking them all the way through as they will be going into the slow cooker. Add meatballs to slow cooker, cover and cook for 6 hours.