The night before you roast the pork, season all over with salt and pepper. Using a meat pounder or a rolling pin, pound the meat out to an even 1/2-inch thickness. Cover with plastic wrap and refrigerate (you can skip this step and season the meat right before cooking, but the results of pre-salting are incredibly good).
Combine the olive oil and butter in a skillet over medium heat. Add the onion and celery and cook, stirring occasionally, until the onion is translucent, about 5 minutes. Add the apple, sage, salt, pepper, and cinnamon. Stir and cook until the apples are tender, about 5 minutes. Pour the mixture into a large bowl. In a food processor, pulse the bread until it forms fluffy breadcrumbs. Add the chestnuts and pulse until they’re roughly chopped. Add the breadcrumbs and chestnuts to the onion-apple mixture. Add the egg and chicken broth and stir to mix evenly. Let cool to room temperature. Meanwhile, preheat the oven to 375ºF and set a rack to the middle position.
Unfold the pork loin and lay horizontally on the counter. Spoon the stuffing over the meat, spreading it evenly with a spatula and leaving a 2-inch border along the top edge. Roll the meat up over the stuffing, jelly-roll style, from the bottom to the top. Turn seam-side down. Using a sharp knife, lightly score any extra fat on the surface of the loin in a diamond pattern. Tie the loin up at regular intervals with pieces of twine and transfer it to the roasting pan.
Pour 1/2 cup of the cider in the bottom of the pan and put the pan in the oven. Roast the loin for 30 minutes. Baste the meat with the pan juices, then add another 1/2 cup of cider. Return to the oven and roast, basting regularly, until a quick-read thermometer inserted into the middle of the loin reads 150ºF, 50 to 60 more minutes.
Transfer the meat to a cutting board and cover loosely with aluminum foil. Scrape the pan bottom with a wooden spoon to dislodge any browned bits, then pour the pan juices into a large glass measuring cup and add the remaining 1/2 cup cider. You should have about 1 1/2 cups liquid. Pour 1/4 cup of the cider mixture into a small saucepan over medium-high heat. Add the cornstarch and whisk until it’s dissolved. Add the remaining cider mixture and bring to a simmer. Reduce the heat to medium and cook, gently stirring with the whisk, until the sauce thickens, about 5 minutes. Taste and add salt and pepper as needed. Pour the sauce into a warmed bowl.
Cut the loin into 3/4-inch-thick slices and arrange on a warmed platter. Serve the hot cider sauce on the side.
Reprinted with permission from The Apple Lover's Cookbook: The Classic Guide to Cooking and Eating Apples by Amy Traverso. Copyright © 2020, 2011, by Amy Traverso. Photos Copyright © 2011 by Squire Fox. Published by W.W. Norton & Company, Inc.