In a large bowl, combine ground beef, breadcrumbs, Parmesan, parsley, egg, garlic, salt, and pepper.

Mix until well combined, but do not overwork the meat.

Form the mixture into meatballs, about 1 to 1.5 inches in diameter.

Place the meatballs in the slow cooker, ensuring they are not overcrowded.

Pour marinara sauce over the meatballs, ensuring they are well coated.

Sprinkle Italian seasoning over the top for added flavor.

Cover the slow cooker with the lid.

Set the slow cooker to low for 6 hours or high for 3 hours.

Check the meatballs for doneness; they should be cooked through and tender.

Serve hot with your choice of sides or in a sub sandwich.