Remove giblets and neck from turkey. Reserve giblets for Lemon-Parmesan Stuffing. Reserve neck for another use. Rinse turkey thoroughly with cold water; pat dry.
Combine butter and lemon juice; stir well with a wire whisk. Carefully loosen skin from turkey at neck area, working down to breast and thigh area. Rub two-thirds of lemon butter mixture under skin. Place 8 to 10 sage leaves between skin and meat. Rub inside of body and neck cavities with lemon halves. Sprinkle cavities evenly with 1/2 teaspoon salt and 1/4 teaspoon pepper.
Lightly stuff Lemon-Parmesan Stuffing into body and neck cavities of turkey. Place turkey on a lightly greased rack in a roasting pan, breast side up. Tie legs together with heavy string. Lift wingtips up and over back, and tuck under bird.
Coat turkey with remaining one-third lemon butter mixture; sprinkle with remaining 1/2 teaspoon salt and 1/4 teaspoon pepper. Pour wine into pan. Insert a meat thermometer into meaty portion of thigh, making sure it does not touch bone.
Bake, uncovered, at 325° for 3 1/2 hours on bottom oven rack or until thermometer inserted into thickest part of leg registers 180° and center of stuffing reaches 165°. Let turkey stand 15 minutes before carving.
While turkey stands, pour pan drippings through a wire-mesh strainer into a large saucepan, discarding solids. Combine flour and 3/4 cup pan drippings, stirring until mixture is smooth; add to remaining pan drippings in saucepan, adding enough water to measure 2 1/4 cups, if necessary. Bring mixture to a boil, stirring constantly; reduce heat, and simmer, uncovered, 5 minutes or until mixture is thickened, stirring often. Serve with turkey. Garnish, if desired.