Preheat oven to 375°F (190°C) and lightly grease a loaf pan.
In a large mixing bowl, combine ground beef, Italian sausage, breadcrumbs, Parmesan, milk, eggs, onion, garlic, oregano, salt, and pepper; mix until just incorporated.
Place half of the meat mixture into the loaf pan, creating a slight indentation down the center.
Fill the indentation with 1 cup of mozzarella cheese.
Place the remaining meat mixture over the cheese and press edges firmly to seal the cheese inside.
Spread 1/2 cup of marinara sauce over the top of the meatloaf.
Bake for 50 minutes or until the internal temperature reaches 160°F (71°C).
Top with the remaining marinara sauce and the remaining 1/2 cup of mozzarella cheese.
Bake for an additional 10 minutes until the cheese is melted and bubbling.
Allow the meatloaf to rest for 10 minutes before slicing and garnishing with fresh parsley.