Preheat oven to 350°. Grease a 9x13-inch baking dish with butter and set aside.
Tear bread into 2-inch chunks. Working in three batches, pulse bread in food processor just enough so that you have some finely ground pieces and some 1-inch chunks, 15 to 20 seconds. Divide between two large rimmed baking sheets; toast in oven until lightly browned, 10 to 15 minutes. Set aside.
Meanwhile, in a 12-inch skillet over medium heat, cook pancetta or bacon until crisp, 10 to 12 minutes. Remove with a slotted spoon and drain on paper towels. Drain fat into a glass measuring cup. Return 3 tablespoons fat to pan; discard the rest. Add onions, celery, apples, rosemary, salt, and pepper; cook, stirring often, until onions are translucent and lightly golden, about 10 minutes. Remove from heat and set aside.
Transfer toasted breadcrumbs to a large bowl. In a small bowl, whisk together cider, broth, and eggs. Pour over breadcrumbs; then add chestnuts and onion/apple mixture. Crumble pancetta or bacon and add to the mixture. Use clean hands to gently toss all together. Transfer to prepared baking dish and bake until browned on top, about 35 minutes.
Chestnut dressing is, well, an old Thanksgiving chestnut, and for good reason. We love the way the sweet nuts pair with the onions and rosemary. We added even deeper flavor with sweet apples and salty pancetta. This dressing is baked outside the bird, to let the meat cook more evenly and eliminate any food-safety concerns.