Preheat the smoker to 250°F. Use a mild wood such as apple or cherry so the smoke complements the Italian seasoning without overpowering it.

Heat the olive oil in a skillet over medium heat. Add the chopped spinach and cook for 1 to 2 minutes, stirring, until wilted. Transfer it to a plate and let it cool. Once cool, gently press out excess moisture.

In a large bowl, combine the ground beef, ground pork, egg, breadcrumbs, milk, Parmesan, garlic, Italian seasoning, salt, and black pepper. Mix gently just until combined to keep the meatloaf tender.

Place a large sheet of parchment on the counter. Shape the meat mixture into a rectangle about 10 inches long and 7 inches wide. Spread the cooled spinach down the center, leaving a border around the edges. Arrange the mozzarella cubes over the spinach.

Lift the parchment to bring the long edges of the meat mixture together over the filling. Pinch the seam and ends closed, then gently shape the loaf so the filling is fully enclosed. Transfer it seam-side down to a smoker-safe pan.

Smoke the meatloaf at 250°F until the center of the meat reaches 160°F on an instant-read thermometer, about 2 to 2½ hours. Spoon the marinara sauce over the top during the final 30 minutes of smoking.

Remove the meatloaf from the smoker and let it rest for 10 minutes before slicing. This helps the loaf hold its shape and keeps the mozzarella center from spilling out too quickly.