Preheat oven to 350 degrees. Using a chestnut knife or a small paring knife, make an incision about 1/8 inch deep through the shell and into the flesh of the chestnut almost all the way around the circumference of the nut. Transfer to a chestnut pan or rimmed baking pan. Roast in the oven until the chestnuts are tender, about 35 minutes. Turn the oven off. Leaving the pan with the chestnuts in the oven, remove several at a time. Working quickly, place 1 chestnut in a towel, and, holding both, peel the chestnut while still hot. Remove and discard the shells and inner skin; coarsely chop, and set aside.
Remove crusts from bread, and set aside. Cut bread into 1-inch cubes. Place in a single layer on two baking sheets, and toast in the oven until dry, 5 to 7 minutes. Set aside to cool. Place reserved crusts in the bowl of a food processor, and pulse until coarse crumbs are formed.
Place prunes and apple cider in a small saucepan, and bring to a boil over medium-high heat. Reduce to a simmer, and cook until all liquid has been absorbed, 20 to 25 minutes. Set aside.
Melt the butter in a large skillet over medium heat, and add the chestnuts, red onion, half the celery, and half the apples. Cook until onion is translucent, about 7 minutes. Set aside to cool.
In a large bowl, combine bread cubes and crumbs, prune mixture, chestnut mixture, remaining celery, apples, eggs, heavy cream, and sage. Stir to combine. The juices from the brine will season the stuffing; stir before serving.
Stuffing can be baked in turkey until its temperature reaches 165 degrees. Excess stuffing can be seasoned with salt and pepper and baked in a buttered baking dish, covered, at 350 degrees for 30 minutes and then uncovered for an additional 10 minutes.