1. Preheat the oven to 425°. Cut along both sides of the wishbone and remove it. Tuck 1 wedge of onion and 1 wedge of apple in the neck cavity along with 1 sprig each of sage, thyme, rosemary and oregano. Fold the skin under the turkey. Season the main cavity with salt and pepper and stuff it with 3 onion and 3 apple wedges, 2 garlic cloves and the remaining herb sprigs. Tie the legs together.
2. Set the turkey in a large roasting pan and season with pepper. Arrange the bacon strips over the breast and legs and roast for 1 hour. Remove the bacon and spoon off some of the fat from the roasting pan. Add the remaining onion and apple wedges and garlic cloves to the pan and turn to coat with fat. Roast the turkey for 45 minutes longer or until an instant-read thermometer inserted in the thickest part of the thigh registers 160°; baste occasionally. Transfer the turkey to a platter, cover loosely with foil and let rest for at least 30 minutes.
3. 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.
4. In a bowl, stir the flour into the remaining 1/4 cup of cider, then whisk the paste into the gravy. Bring to a boil; simmer until slightly thickened, about 5 minutes. Stir in the apple and onion puree and the vinegar and season with salt and pepper. Garnish the turkey with herb sprigs and serve with the gravy.