Preheat the oven to 350 degrees. Slice the potatoes as thinly as possible on a mandolin slicer. Heat the duck fat to liquify. Mix the potatoes, salt, pepper, and duck fat. Make sure the potatoes are evenly coated. Layer the mixture in a loaf pan. Bake for 1 hour and 10 minutes or until the potatoes are tender and browned. Cool for 10 minutes, slice and serve.