Let the turkey sit at room temperature, 30 minutes. Position an oven rack in the lowest position (remove the other racks); preheat to 350 degrees F.
Remove the neck and giblets from the turkey; set aside for the gravy. Finely grate the zest from all 4 lemons; combine with the peppercorns and 1 tablespoon salt. Pat the turkey very dry with paper towels. Season the inside with about one-quarter of the lemon-pepper mixture, then rub the rest all over the skin. Cut one of the lemons into quarters; stuff inside the cavity along with the onion, celery and thyme sprigs. Tie the legs together with kitchen twine. Put the turkey on a rack set in a large roasting pan and tuck the wings under the body.
Cook the bacon in a large skillet over medium heat until crisp, 7 to 8 minutes. Transfer to paper towels to drain, reserving the drippings in the skillet. Chop the bacon; set aside for the gravy. Add the butter to the skillet with the drippings and stir until melted, then pour into a bowl and stir in the juice of 2 lemons. Let the lemon butter cool slightly, then brush all over the turkey. Transfer to the oven and roast 1 hour. Meanwhile, make the Classic Gravy.
After the turkey has roasted 1 hour, baste it with the drippings. Continue roasting, basting every 30 minutes, until the skin is golden brown and a thermometer inserted into the thigh registers 165 degrees F, about 2 more hours.
Transfer the turkey to a cutting board and let rest 30 minutes before carving; reserve the drippings for the gravy. Stir the bacon into the finished gravy.
Prepare the stock: Melt 2 tablespoons butter in a large saucepan over medium heat. Add the turkey neck and giblets; cook, turning, until browned, about 5 minutes. Add the onion, carrot, celery, thyme and bay leaves; stir to coat. Add the wine and bring to a boil, scraping up any browned bits from the bottom of the pan. Cook until reduced by half, 2 to 3 minutes. Add the broth, reduce the heat to low and simmer about 1 hour. Strain through a fine-mesh sieve into a large measuring cup; reserve the saucepan. You should have 7 cups stock-if you're short, add more broth.
Melt the remaining 8 tablespoons butter in the reserved saucepan over medium heat. Add the flour and whisk until smooth and bubbling, about 2 minutes. Gradually whisk in the 7 cups stock; bring to a simmer and cook, whisking occasionally, until thickened, about 10 minutes. Set aside until the turkey is done.
Pour the turkey pan drippings into a fat separator and let stand until the fat rises to the top. Discard the fat (or drizzle on your stuffing). Whisk the defatted drippings into the gravy; season with salt and pepper. Reheat before serving.