Melt the butter in a medium saucepan over medium heat. When it foams, whisk in the flour until combined. Cook until the flour loses its raw flavor and turns peanut color, about 3 to 5 minutes. Whisk in the chicken broth until the mixture is smooth. Bring to a simmer, then reduce the heat to low and cook until it becomes a light tan color.
Meanwhile, remove the turkey from roasting pan and set aside to rest. Discard all but 1 tablespoon of the drippings from roasting pan. Put the pan on the stovetop over medium heat. When pan juices are hot, add the sage leaves and fry until fragrant, about 1 minute. Remove the neck bone. Add a big splash of apple juice and scrape up the browned bits at the bottom of the pan.
Strain this liquid into the thickened chicken broth mixture and season with salt, pepper and nutmeg. Whisk to combine. Bring to a simmer and cook for a few minutes until thick and creamy, about 3 to 5 minutes. Serve with your turkey and sides.