Remove rack from roasting pan and pour pan juices through a sieve into a 1-quart glass measure. Place roasting pan across two burners over high heat, add cider and deglaze pan, stirring and scraping up any brown bits on the bottom of the pan. Cook until reduced to 1/2 cup, about 5 minutes. Pour cider through sieve into glass measure with pan juices, skim fat, reserving 1/4 cup. Add enough turkey stock to drippings to equal 4 cups.

Using a large sauté pan over medium heat, add butter. Sauté onions, stirring occasionally, until golden brown, about 10 minutes. Add sage and cook, 1 minute. Add turkey stock mixture and any turkey juices accumulated on platter and bring to a boil. Using a small bowl whisk together flour and reserved 1/4 cup fat. Whisk into gravy, reduce heat and simmer, whisking occasionally, until thickened, about 10 minutes. Taste and adjust for seasoning with salt and pepper.