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1
Preheat oven to 350 degrees F (175 degrees C).
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2
Bring a large pot of salted water to a boil. Cook shell pasta in the boiling water, stirring occasionally, until cooked through but firm to the bite, about 8 minutes. Drain.
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3
Whisk flour into 3 tablespoons of turkey broth in a bowl until the mixture is smooth and free of lumps; transfer to a saucepan. Whisk turkey gravy, sherry, and 1/2 cup turkey broth into the flour mixture and bring to a boil; reduce heat to medium and cook until thick, about 5 minutes, whisking constantly. Remove from heat.
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4
Process turkey and stuffing in a food processor until finely ground, about 1 minute. Add ricotta cheese, Swiss cheese, Parmesan cheese, and 1/2 cup mozzarella cheese to the food processor and blend until filling is thoroughly combined, about 1 more minute. Add egg and pulse 1 or 2 times to combine. Transfer turkey and cheese filling to a bowl and gently stir in 1/2 cup more mozzarella cheese and chopped parsley.
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5
Pour about 1/4 of the turkey gravy mixture into the bottom of a 9x13-inch baking dish and swirl to coat the bottom. Stuff shells with about 1 1/2 tablespoons of the turkey filling and arrange in the baking dish. Pour remaining turkey gravy over the shells and sprinkle remaining 1/2 cup mozzarella cheese over the gravy. Cover the baking dish tightly with aluminum foil.
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6
Bake in the preheated oven until the filling is set, the casserole is bubbling, and the cheese topping has melted, about 45 minutes.
