In a large bowl, mix ground beef and ground pork.
Add breadcrumbs, Parmesan cheese, egg, garlic, oregano, basil, salt, and pepper.
Knead the mixture until just combined; avoid overmixing.
Form the mixture into meatballs, about 1.5 inches in diameter, and push a cube of mozzarella into the center of each.
Place formed meatballs into the slow cooker.
Pour marinara sauce over the meatballs, ensuring they are all covered.
Cover the slow cooker and cook on low for 6-8 hours or on high for 3-4 hours.
Once cooked, serve the meatballs with extra sauce over pasta or in subs.