In a large bowl, combine the ground chicken, breadcrumbs, Parmesan cheese, parsley, egg, garlic, Italian seasoning, salt, and black pepper. Mix until well combined.
Form the mixture into meatballs, about 1 to 1.5 inches in diameter. You should get around 20 meatballs.
Place the meatballs in the slow cooker. Pour the marinara sauce over the meatballs, ensuring they are well coated.
Cover and cook on low for 4 to 6 hours or on high for 2 to 3 hours, until the meatballs are cooked through and reach an internal temperature of 165°F.
About 15 minutes before serving, sprinkle the shredded mozzarella cheese on top of the meatballs. Cover and let the cheese melt.
Serve the meatballs hot, garnished with fresh basil leaves if desired.