Preheat an oven to 180 C / Gas 4. Grease a baking dish.
Fill a large pot with lightly salted water and bring to the boil. Add pasta and return to the boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 13 minutes. Drain well in a colander set in the sink.
Place turkey and stuffing in food processor, and pulse until finely ground and combined. Place the minced turkey and stuffing mixture in a bowl along with cream cheese, Parmesan cheese, mayonnaise, and half of the mozzarella. Mix until well blended.
Spread some of the gravy (or chicken soup) on the bottom of the prepared dish. Stuff the pasta shells with the turkey mixture and place in the dish in tight rows. Top with the remaining gravy and the remaining mozzarella cheese.
Bake, covered, for 45 minutes. Uncover and cook for an additional 10 minutes until top is browned and bubbly. Allow to cool for 5 minutes before serving.