Preheat the oven to 375°F / 190°C.

Place the ground beef, Italian sausage, onion, garlic, bread crumbs, eggs, ½ cup Parmesan, Italian seasoning, and parsley in a large bowl. Add the milk, salt, and pepper, then mix until the ingredients are evenly incorporated.

Transfer the meat mixture to a baking sheet and form it into a 9x12-inch rectangle.

Create a lengthwise well through the center of the meat mixture. Add the mozzarella, spinach if using, and ¼ cup marinara sauce to the well.

Bring the meat over the filling and press the edges together firmly to enclose it completely. Shape the sealed mixture into a loaf.

Coat the top of the loaf with the remaining ½ cup marinara sauce and scatter ¼ cup Parmesan cheese over it.

Bake for 45-55 minutes, or until the internal temperature reaches 160°F / 70°C.

Remove the meatloaf from the oven and allow it to rest for 10 minutes before slicing.

Slice the rested meatloaf and serve with extra marinara sauce if desired.