Reserve 1 cup apple cider for basting; cover and refrigerate. In 6-quart bowl or stockpot, stir remaining cider, the brown sugar, kosher salt, 1/4 cup fresh sage and the peppercorns until salt is dissolved. Add turkey breast. Cover; refrigerate at least 12 hours but no longer than 24 hours.
Heat oven to 325°F. Remove turkey from brine; rinse thoroughly under cool running water, and pat dry. Discard brine.
Place turkey, skin side up, on rack in large shallow roasting pan. Insert ovenproof meat thermometer so tip is in the thickest part of breast and does not touch bone. Roast uncovered 1 hour.
In 1-quart saucepan, heat reserved apple cider, the butter, 1 tablespoon fresh sage and the garlic over medium heat until butter is melted and mixture is hot. Roast turkey about 1 hour longer or until thermometer reads 165°F and juice of turkey is clear when center of thickest part is cut, basting turkey generously with apple cider mixture and pan juices every 15 minutes. Remove turkey from oven, and let stand 15 minutes for easier carving.
Meanwhile, pour pan drippings and scrapings into measuring cup; let stand 5 minutes. Skim 4 tablespoons fat from top of drippings, and place in 2-quart saucepan; skim and discard any remaining fat. Add enough water to remaining drippings to measure 2 cups; set aside.
Stir 1/4 cup flour into fat in saucepan, using wire whisk. Cook over medium heat, stirring constantly, until mixture is smooth and bubbly; remove from heat. Gradually stir in reserved 2 cups drippings and the salt. Heat to boiling, stirring constantly. Boil and stir about 1 minute or until gravy thickens.