Heat the oven to 375°F. Line a rimmed baking sheet with parchment paper.
In a large mixing bowl, combine the ground turkey, egg, breadcrumbs, Parmesan, milk, garlic, Italian seasoning, salt, and black pepper. Mix gently just until everything is evenly combined.
Place half of the turkey mixture on the prepared baking sheet and shape it into a compact loaf. Arrange the mozzarella cubes down the center, leaving a border around the edges. Cover with the remaining turkey mixture and pinch the seams closed so the cheese is sealed inside.
Spoon ½ cup of the marinara sauce over the top of the loaf. Bake until the center reaches 165°F on an instant-read thermometer, about 45 to 55 minutes.
Let the meatloaf rest for 10 minutes before slicing. Warm the remaining ¼ cup marinara sauce and serve it alongside the slices. Sprinkle with chopped parsley.