For the duck:
Score the skin of the duck breast, making sure you do not pierce the meat. Add oil to an ALL-CLAD 3 qt sauté pan and heat over moderate heat. Season the duck with salt and pepper. Place the duck skin side down and let cook for about 8 minutes. You want the fat to render from the skin. Once the duck has browned nicely place in a 350 degree oven for about 5 minutes. Take out of oven and place skin side up and let rest.
In a large pot add potatoes and cover with cold water and add a generous amount of salt. Bring to a boil, once boiling, cook for 3 minutes and drain and set aside. Using the same pan, remove half of the duck fat from the 3 qt All-Clad Saute pan. Add onions, carrots, celery and garlic. Cook, occasionally stirring for 5 minutes, add butter, sage and boiled potatoes and continue cooking for another 5 minutes. Add chestnuts and sauté for 3 minutes, add brussels sprouts and sauté for 2-3 minutes. Season to taste with salt & pepper.
Dried cherry sauce
Preheat oven to 350°
In a small bowl soak dried cherries in Armagnac and set aside.
In 5-QT pot over moderate heat 1 tablespoon add olive oil and sauté until soft, about 3 minutes. Add red wine and port, and reduce to ¾ of the way. Add chicken stock and reduce by half, add demi-glace and season. Stir in cherries and Armagnac, cook slowly for 10 minutes over medium heat. At the last minute add fois grais and poach for 2 minutes.
Slice the duck breast on a bias and serve half a breast per plate with sauce.