Preheat the oven to 350 degrees F. Spray a 13 by 9-inch pan with the vegetable oil cooking spray.
Cook the pasta in plenty of boiling, salted water until it has softened a bit but is still not quite cooked all the way through. (The pasta will absorb the liquid from the sauce as it finishes cooking in the oven.) Drain, rinse with cold water to stop the cooking, drain again, and set aside.
Melt 2 tablespoons butter in a large skillet over medium-high heat and add the turkey. Cook, breaking up the meat into large clumps, until it is cooked through, about 10 minutes. Season it with a little salt and pepper. Remove the meat with a slotted spoon to drain any excess liquid from the meat.
Let the pan cool a bit, then add the remaining butter and melt it over medium heat. Add the flour and whisk to combine. Whisk constantly until the flour has cooked but not browned, about 2 minutes. Add the milk slowly, whisking to avoid creating any lumps. Add the chicken stock, cayenne, and nutmeg and cook, stirring frequently, until the mixture begins to thicken, about 5 to 10 minutes. Stir in the sage and season with some salt and pepper. Fold in the turkey and pasta, making sure it is all well coated with the sauce. Pour into the prepared pan.
To make the topping, combine the bread crumbs and Parmesan and moisten with the melted butter. Spread topping evenly over the tetrazzini, put the pan into the oven, and cook until the top is golden brown and the mixture is bubbling, about 30 minutes. Let rest for 10 minutes before serving.
You can also fill 8 individual greased 10 to 12-ounce ramekins with the tetrazzini and topping. These will bake in about 20 minutes. These can be frozen: Make sure they are cool, then add 1 tablespoon milk, chicken stock, or water to each ramekin. Wrap them well in plastic wrap and put them into the freezer. Poke a hole in the plastic wrap and microwave on defrost for 5 minutes, then microwave on high heat for about 8 to 10 minutes or until the tetrazzini is hot and bubbling.