Transfer the onion, apple and garlic in the roasting pan to a food processor and puree. Set the pan over 2 burners on moderately high heat; when the juices begin to sizzle, stir in 1 cup of the apple cider and cook, scraping up the browned bits, until reduced by half, about 5 minutes. Strain the pan juices into a medium saucepan. Add the turkey stock and boil until reduced to 2 cups. Skim off the fat.