After roasting your turkey, transfer it to a cutting board and cover with foil to let it rest before carving.
Pour the turkey pan drippings from the roasting pan into a gravy separator or a large measuring cup and let stand a few minutes, until the fat separates & rises to the top. Discard the fat and keep the rest of the turkey drippings in a large measuring cup. Now, add enough turkey stock to the pan drippings to equal 3 cups total. Set aside.
Meanwhile, over medium-heat in the roasting pan, melt 4 tablespoons of butter. Stir the flour into the butter to make a thick paste (roux) and cook for 1-2 minutes, stirring constantly.
Slowly whisk the turkey liquid into the roux, then whisk in the apple cider. Continue whisking until all the lumps have dissolved.
Bring the gravy to a simmer and let cook for 10 minutes. Feel free to thin the gravy if necessary with a little more turkey stock until it’s the consistency that you like.
Finally, turn off the heat, stir in the apple brandy and the sage. Taste, and re-season with salt and pepper if necessary. Pour into a gravy boat and serve!