This method is tailored for entertaining, since the duck can go into the oven before guests are seated, and needs only to be sliced to serve. It is also a foolproof technique, delivering properly medium-rare slices.
Preheat oven to 200 degrees. Trim a tablespoon of fat from edges of each breast, and set aside. With a sharp knife score skin of each breast in a crisscross pattern. Rub flesh side of each with 1/4 teaspoon five-spice powder, and season with salt and pepper.
Place a heavy skillet, preferably cast iron, over medium-high heat. Place breasts skin side down in skillet, and sear a few minutes, until skin is nicely browned. Place breasts in a baking dish skin side up. Place in oven for 1 hour.
Meanwhile, melt reserved fat in a saucepan over medium heat. Add shallots, and cook until soft. Add quince slices, remaining five-spice powder and the sugar; cook, stirring, a few minutes, until quince starts to color. Add stock, bring to a simmer and remove 1/2 cup stock to a dish. Add wine to saucepan, cover and cook about 15 minutes, until quince is tender. Whisk hoisin sauce into reserved stock. When quince is tender, stir in hoisin mixture. Simmer 10 minutes. Season with salt and pepper, and set aside.
Remove breasts from oven, and slice on bias. Pour any juices into pan with quince. Arrange duck on a platter. Reheat quince mixture, and spoon around duck. Serve.