Preheat oven to 425F/220C.
Remove duck fat from refrigerator and bring to room temperature.
Cut potatoes into 1-inch chunks, leaving skin intact. (I cut the potatoes in half, then quarter each side.) Place potatoes in a large stock pot and add enough water to cover potatoes. Bring to a boil. When the water comes to a boil, set the timer and boil for 5 minutes.
Drain potatoes in colander. Shake the colander to drain excess water. Set aside for 5 more minutes to continue steaming/draining.
Use the largest rimmed sheet pan that you have. Dump potatoes onto a sheet pan. Drizzle with duck fat, as it should be somewhat liquified by now. Sprinkle with salt and pepper. Mix well with hands and spread out in an even layer onto the sheet pan.
Roast until potatoes are crispy and browned, about 40-55 minutes. About halfway through, scrape up the potatoes with a spatula and turn them to the other side.
Garnish with parsley and a few more grinds of fresh pepper. Serve immediately.