Duck is a notoriously fatty bird. To diminish the fat and produce a crispy skin, begin by trimming the excess fat from the body. Rinse the duck thoroughly, inside and out, and pat dry with paper towels. Season the bird inside and out with a generous amount of salt and pepper.
To prepare the stuffing: Melt the butter in a large skillet over medium heat. Add the onion, celery, celery seed, prunes, apples, and 4 sage leaves; season with salt and pepper; saute for 10 minutes until soft. Add the bread cubes and toss the mixture together to combine. Put the stuffing in a large mixing bowl and moisten it with a squeeze of lemon and the heavy cream; give it another toss and season with salt and pepper. Spoon the stuffing into the duck cavity. Rip off a foot long piece of aluminum foil and lay it on an insert rack fitted in a roasting pan, let a bit of the foil hang off the end. Lay the duck, breast-side up, on the foil; tuck the wing tips back under the duck, and fold the excess foil over the end of the duck with the stuffing. The foil will protect the stuffing from burning and falling into the delicious duck fat.
Preheat the oven to 375 degrees F.
Roast the duck for 21/2 to 3 hours, rotating the pan every 20 minutes or so. It may seem like a bother, but it's the best way to ensure an even crispy skin. The legs will wiggle easily when the bird is done and an instant-read thermometer will register about 180 degrees F when inserted into the thigh.
Take the insert rack out of the pan and set the duck on a cutting board to let it rest before carving. Now you have a whole lot of duck fat in the bottom of the roasting pan. Pour out all but a couple of tablespoons of the duck fat into a container and reserve.
For the gravy: Place the roasting pan, with the couple of tablespoons of duck fat, on the stove over 2 burners set on medium heat. Sprinkle in the flour and stir to make a paste. Crank the heat up to high and add the wine, cook and stir, scraping the bottom of the pan, until the liquid is reduced slightly. Add the chicken stock and remaining 4 sage leaves, season with salt and pepper. Cook and stir for 1 to 2 minutes until the gravy has thickened slightly.