Heat the oven to 375°F. Line a baking sheet with parchment paper.
Warm the olive oil in a skillet over medium heat. Add the spinach and cook, stirring, until wilted. Transfer it to a plate and let it cool. Squeeze out any excess liquid, then roughly chop the spinach.
In a large bowl, combine the ground turkey, egg, breadcrumbs, milk, Parmesan, garlic powder, oregano, salt, and black pepper. Mix gently just until combined.
Place the turkey mixture on the prepared baking sheet and shape it into a rectangle about ½ inch thick. Leave a small border around the edges. Spread the chopped spinach down the center, then top it with the mozzarella cubes.
Fold the long sides of the turkey mixture over the filling and pinch the seams and ends closed. Shape it into a compact loaf with the seam underneath. Spread the ketchup over the top.
Bake until the center reaches 165°F on an instant-read thermometer. Rest the meatloaf for 10 minutes before slicing so the filling settles and the loaf stays tender.