How to Make It
Preheat the oven to 450°. Melt the butter in a large skillet. Add the leeks and celery and cook over moderate heat, stirring, until softened, about 10 minutes. In a measuring cup, add enough turkey stock to the oyster liquor to make 2 cups.
Stir the flour into the leek mixture and cook for 1 minute. Stir in the turkey stock mixture and bring to a simmer. Add the turkey, cream and parsley and season with salt and pepper. Butter a 12-by-8-by-2-inch baking dish and spread the filling in it. Arrange the oysters on top.
In a large bowl, stir together the flour, baking powder and salt. Quickly rub in the butter with your fingertips. Stir in half of the Parmesan. Mix the milk and cream and gently stir the liquid into the dry ingredients until just combined.
Drop rounded teaspoons of the biscuit topping onto the filling; the topping will spread as it bakes. Sprinkle with the remaining cheese, set the dish on a baking sheet and cook for about 30 minutes, or until the topping is golden and crusty. Let the potpie stand for 10 minutes before serving.