In a large bowl, combine ground beef, breadcrumbs, Parmesan, parsley, egg, garlic, salt, pepper, and Italian seasoning.
Mix the ingredients until well combined, but do not overmix.
Form the mixture into meatballs, about 1 inch in diameter.
Place the meatballs in the crockpot in a single layer if possible.
Pour marinara sauce evenly over the meatballs.
Cover the crockpot and set it to low heat, cooking for 6-8 hours.
Check for doneness; meatballs should be cooked through and reach an internal temperature of 160°F.
Once cooked, gently stir the meatballs in the sauce to coat.
Serve hot, garnished with additional parsley if desired.