1. For the biscuit rounds: Whisk the flour, baking powder, salt and sugar together in a large bowl. Using your fingers, rub 3 tablespoons of the butter into the flour until the mixture is sandy. Work in the remaining 5 tablespoons butter until it forms pea-size pieces. Stir the lemon zest into the milk; stir the milk into the flour mixture to make a soft dough.
2. Lightly flour a work surface. Turn the dough out onto the work surface and pat into a 1/2-inch-thick rectangle. Fold the dough in thirds as you would a business letter, then pat into a 1/2-inch-thick disc. Cut out 8 biscuit rounds using a 3-inch cutter, set on a tray, cover with plastic wrap and refrigerate until ready to use.
3. For the smoked turkey and file gumbo: Add the vegetable oil to a soup pot over medium heat. Stir in the flour and cook, stirring constantly, until the mixture thickens to a paste and turns medium brown, about 10 minutes. Add the celery, onions and bell peppers and cook until softened, about 5 minutes. Add the chicken stock, turkey meat, Cajun seasoning and bay leaves. Bring to a boil, lower the heat and simmer until the soup thickens slightly, about 30 minutes.
4. Preheat the oven to 450 degrees F.
5. Add the file powder, salt, black pepper, cayenne and the oysters with any reserved liquor to the gumbo pot. Simmer until the mixture thickens slightly, about 5 minutes.
6. To assemble the pot pie: Spoon the rice in the bottom of the baking dish. Top with 8 cups of gumbo. Arrange the biscuit rounds on top of the gumbo and brush lightly with milk. Put the baking dish on a baking sheet and bake until golden brown, about 20 to 25 minutes. Let the potpie rest for 5 minutes before serving.