In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, Parmesan cheese, parsley, egg, garlic, Italian seasoning, salt, and black pepper. Mix until just combined; do not overmix.
Form the mixture into meatballs, about 1.5 inches in diameter. You should have approximately 24 meatballs.
Optionally, place the meatballs on a baking sheet and bake at 400°F (200°C) for 15 minutes for a slightly crispy exterior.
In the slow cooker, pour in 1 cup of marinara sauce to coat the bottom.
Carefully place the meatballs into the slow cooker and pour the remaining marinara sauce over the top.
Cover and cook on low for 6 hours, or cook on high for about 3 hours, until the meatballs are cooked through and tender.
Once cooked, garnish with fresh basil before serving.