1. Cook sausage in a large nonstick skillet over moderately high heat, stirring and breaking up lumps, until no longer pink, about 5 minutes. Drain in a colander set over a bowl, reserving skillet (discard fat in bowl).
2. Cook onion in reserved skillet over moderately high heat, seasoning with salt and pepper, stirring, until golden brown, about 8 minutes. Add apple and 1 tablespoon sage; cook, stirring, until apple is just softened, about 3 minutes. Stir in cider and vinegar; simmer until liquid is almost evaporated but mixture is not dry; about 1 minute. Transfer to a bowl and let cool to room temperature, about 20 minutes.
3. Meanwhile, preheat oven to 375 degree F and arrange rack in lower third of oven; lightly coat a 9-inch nonstick springform pan with cooking spray.
4. Unfold 1 piecrust on a lightly floured surface. Using a floured rolling pin, roll out into a 13-inch round and fit into prepared pan, slightly pleating sides. Prick bottom with a fork and freeze until firm, about 15 minutes. Roll out remaining piecrust as above, then, using a fluted pastry cutter, cut into 1/2-inch-wide strips; transfer to a cooking sheet and freeze until firm, about 15 minutes.
5. Line piecrust in springform pan with foil and fill with pie weights or dried beans. Bake until crust is pale golden and set, 12 to 15 minutes. Carefully remove foil and pie weights; transfer crust in pan to rack to cool.
6. meanwhile, whisk together milk and 4 eggs. Stir in sausage, onion mixture, remaining 1 tablespoon sage and the cheese.
7. Lightly beat remaining egg in a small bowl. Wrap outside of springform pan with foil to catch any leaks. Pour filling into crust and decoratively arrange enough pastry strips over top to form an open basket weave. Trim ends flush with edge of bottom crust and brush strips with beaten egg.
8. Bake pie in a baking pan in lower third of oven until top is golden brown and filling is set, 35 to 40 minutes. Remove pie from baking pan and cool on rack 20 minutes. Remove foil and sides of pan. Garnish pie with sage leaves and serve slightly warm or at room temperature. Makes 8 servings.