In a large mixing bowl, combine ground beef, breadcrumbs, grated Parmesan, egg, garlic powder, onion powder, oregano, salt, and pepper.
Take a handful of the mixture and flatten it in your hand. Place a tablespoon of mozzarella cheese in the center, then fold over the meat to encase the cheese completely, forming a ball.
Set the prepared meatballs aside on a platter while you prepare the slow cooker.
Pour half of the marinara sauce into the bottom of the slow cooker, making a layer to prevent sticking.
Carefully place the meatballs into the slow cooker, arranging them in a single layer.
Pour the remaining marinara sauce over the meatballs, ensuring they are well covered.
Cover the slow cooker and set it on low for 6 to 8 hours, or high for 3 to 4 hours.
Once cooked, carefully remove the meatballs with a slotted spoon to a serving platter.
Serve the meatballs with a generous ladle of marinara sauce and sprinkle with additional Parmesan if desired.
