Preheat oven to 425 degrees F.

Rinse turkey and pat dry inside and out. Season turkey inside and out with salt and pepper and pack neck cavity with 1 lemon wedge, 1 sage sprig, and 1 bread slice. Fold neck skin under body and fast en with a skewer. Fill body cavity with apple, onion, 3 sage sprigs, and remaining lemon wedge and bread slice and truss turkey.

Rub turkey with remaining 3 sage sprigs and arrange sprigs on a rack set in a roasting pan. Spread turkey with butter and arrange on rack in roasting pan. Roast turkey in middle of oven 30 minutes.

Reduce temperature to 325 degrees F. and baste turkey with pan juices. Add water to roasting pan and roast turkey, basting every 20 minutes, 2 1/2 to 3 hours more, or until a meat thermometer inserte d in fleshy part of a thigh registers 180 degrees F and juices run clear when thigh is pierced. Transfer turkey to a heated platter, reserving juices in roasting pan, and discard string. Keep turkey warm, covered loosely with foil.

Make gravy:

Skim fat from pan juices, reserving 1/4 cup fat, and on top of stove deglaze pan with Sherry over moderately high heat, scraping up brown bits. Stir in cider. Bring Sherry mixture to a boil and remov e pan from heat.

In a heavy saucepan whisk together reserved fat and flour and cook roux over moderately low heat, whisking, 3 minutes. Add Sherry mixture and 2 cups stock or broth in a stream, whisking to prevent lumping, and simmer, whisking occasionally, 10 minutes. Whisk in additional stock or broth to thin gravy if desired. Season gravy with salt and pepper and transfer to a heated gravy boat.

Garnish turkey with sage.