For the galette, blot potatoes dry on paper towels. Combine in a large bowl with onions and a liberal amount of salt and pepper, and mix well. Heat fat until hot in a 10-inch nonstick skillet, over medium high heat. Add potatoes, shaking the pan and turning them to cover evenly with fat. Once a few slices begin to brown, press potatoes with a spatula to flatten into a disk. Adjust heat to medium low, cover tightly, and cook until several slices are golden brown on the bottom, about 5 to 7 minutes. Carefully lift off cover so condensation does not fall on potatoes, and wipe dry. Mix cooked slices into other potatoes and than add shaved truffles, if desired. Replace cover, and cook until bottom of galette is golden brown, about 5 minutes longer. Shake galette onto a plate, cover with a second plate of the same size, invert then slide galette back into pan. Do not worry if some slices need rearranging. Flatten potatoes again, and cover. Cook 10 to 12 minutes longer, removing lid after 5 minutes, or until potatoes are golden brown, turning heat up slightly if needed to color potatoes. Turn galette again, if necessary, and cook uncovered for a few minutes longer, or until golden brown. Galette may be loosely tented with aluminum foil and kept warm in the oven or on top of the stove. Slide galette onto a flat plate. Season with salt and pepper. Combine parsley with garlic, and sprinkle over potatoes. Cut into 8 wedges and serve.
For the duck, score the skin. Season both sides of duck breasts with salt and pepper. Heat 2 heavy skillets until medium hot, over medium high heat. Put duck breasts in pans, skin side down, without any butter or fat. Cook for 8 minutes, checking to avoid burning. Remove fat as it accumulates. Flip breasts over, lower heat to medium and cook 4 minutes longer. Remove duck from heat and keep warm in one of the skillets. Meanwhile, heat the port wine in a saucepan to a boil. Add the diced plums and 2 tablespoons of demi-glace and simmer, partially covered, until the plums are cooked and tender (10 to15 minutes.) Puree with a hand mixer until smooth. Adjust seasoning and keep warm. Heat 2 tablespoons of sugar until lightly caramelized. Add the Armagnac in which prunes have marinated for no less than 15 days. Reduce by half. Add 3 tablespoons prune juice. Reduce until almost syrupy. Strain and season. Keep warm. Cut the breasts across the grain into 1/4-inch slices. Place the plum compote in the center of the warm plates, fan the duck slices on top and drizzle the reduction over it. Serve immediately with a potato galette. Use the prunes to make prunes in Armagnac ice cream, or prune tart, or eat them as is as a digestif.