Mix the Meatball Mixture. In a large bowl, combine the ground beef, breadcrumbs, egg, Parmesan cheese, minced garlic, finely chopped onion, Italian seasoning, salt, and black pepper. Mix gently until everything is just combined. Be careful not to overmix, which can make the meatballs dense.
Shape the Meatballs. Roll the mixture into evenly sized meatballs. Keeping them roughly the same size helps them cook evenly.
Brown if Desired. For extra flavor, you can quickly brown the meatballs in a skillet with a little olive oil before adding them to the slow cooker. This step is optional, but it gives them a deeper savory taste.
Add Sauce to the Slow Cooker. Pour a little marinara sauce into the bottom of the slow cooker. Add the meatballs in a single layer if possible, then pour the remaining sauce over the top.
Cook Low and Slow. Cover and cook on LOW for 5 to 6 hours or on HIGH for 2 1/2 to 3 hours, until the meatballs are fully cooked and tender.
Garnish and Serve. Top with fresh parsley and extra Parmesan if desired. Serve warm over pasta, with bread, or as an appetizer.