Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.

In a large bowl, combine ground beef, Italian sausage, chopped onion, minced garlic, bread crumbs, eggs, 1/2 cup Parmesan cheese, Italian seasoning, parsley, milk, salt, and pepper. Mix gently until just combined - avoid overmixing to keep the meatloaf tender.

Turn the meat mixture out onto the prepared baking sheet and shape it into a rectangle approximately 9x12 inches and about 1/2 inch thick.

Create a well down the center of the rectangle, leaving about 2 inches of meat on each side. Fill the well with shredded mozzarella cheese, chopped spinach (if using), and 1/4 cup marinara sauce.

Carefully fold the long sides of the meat over the filling, pinching the seam firmly to seal with no gaps remaining. Gently roll the loaf so the seam is on the bottom.

Spread the remaining 1/2 cup marinara sauce evenly over the top of the meatloaf, then sprinkle with the remaining 1/4 cup Parmesan cheese.

Bake for 45-55 minutes, or until a meat thermometer inserted into the thickest part reaches 160°F (71°C).

Remove from the oven and let the meatloaf rest for 10 minutes before slicing. This allows the juices to redistribute and makes slicing easier.

Slice and serve with extra marinara sauce if desired.