Preheat your oven to 375°F (190°C).

In a large bowl, mix together ground beef, Italian sausage, onion, garlic, bread crumbs, eggs, Parmesan cheese, Italian seasoning, and parsley.

Add milk, salt, and pepper, then mix until everything is well combined.

On a baking sheet, shape the meat mixture into a rectangular loaf about 9x12 inches.

Create a well in the center of the meatloaf and fill it with mozzarella cheese, spinach (if using), and a spoonful of marinara sauce.

Carefully fold the meat around the filling, sealing the edges to form a loaf.

Spread the remaining marinara sauce on top of the meatloaf and sprinkle with Parmesan cheese.

Bake for 45-55 minutes, or until the internal temperature reaches 160°F (71°C).

Let the meatloaf rest for 10 minutes before slicing.