1Heat oven to 500°F and remove duck fat from fridge. Place potatoes in a large pot and fill with water until potatoes are covered. Add 2 tablespoons salt and the vinegar. Bring water to a boil, reduce to a simmer, and cook 5 minutes. Drain and transfer potatoes to a large bowl.
2Add fat to bowl and toss. Season with salt and pepper to taste, and then toss again. Spread potatoes out in a thin layer on a large rimmed baking sheet.
3Bake for 20 minutes. Remove from oven, flip potatoes over and sprinkle with rosemary and parmesan. Bake another 20 minutes. Remove potatoes from oven, serve and enjoy.