Combine the crushed tomatoes, tomato paste, bay leaves and parmesan rind in a 6-quart slow cooker. Add 1 cup water, 1/4 cup parsley, 2 chopped garlic cloves, 1/2 teaspoon salt and a few grinds of pepper.
Combine the breadcrumbs and milk in a medium bowl; soak 2 minutes. In a separate bowl, combine the beef, eggs, 1 cup parmesan, the remaining 1/4 cup parsley and 2 chopped garlic cloves, 1 teaspoon salt and 1/2 teaspoon pepper. Add the breadcrumb-milk mixture and mix with your hands until combined. With damp hands, shape into 24 meatballs, about 1 1/2 inches each; transfer to the slow cooker.
Add the sausage pieces to the slow cooker, then cover and cook on low, 7 hours. Remove the bay leaves and parmesan rind and stir in the remaining 1/2 cup parmesan. Season with salt and pepper. Serve on the bread and top with mozzarella.