Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a mixing bowl, combine the ground turkey, egg, breadcrumbs, Parmesan, milk, Italian seasoning, garlic powder, salt, and black pepper. Mix just until combined.
In a separate bowl, mix the chopped spinach with the mozzarella cubes, distributing the spinach around the cheese.
Divide the turkey mixture into 8 portions. Press each portion into a muffin cup and make a small well in the center.
Add a little of the spinach and mozzarella mixture to each well. Cover the filling with the turkey mixture and gently seal the edges so the cheese is enclosed.
Spoon a little marinara sauce over each muffin. Bake for 22 to 27 minutes, until the turkey is cooked through and the centers reach 165°F.
Let the muffins rest in the pan for 5 minutes before lifting them out. This helps them hold their shape and gives the filling time to settle.