Peel the potatoes, then cut them into evenly sized 7.5cm / 3-inch chunks to ensure consistent cooking. Preheat the oven to 230°C / 450°F (230°C fan-forced).
Par boiling & roughing up: Bring a large pot of water to a rolling boil and add 1 tablespoon of salt. Add the potatoes and boil for about 10 minutes, just until the edges begin to soften. Drain thoroughly. Shake any remaining water from the pot, return it to the turned-off stove, then tip the potatoes back in and let them sit for 5 minutes so excess moisture can steam away.
Sprinkle over the kosher salt and semolina. Place the lid on the pot and shake vigorously up and down 5 to 7 times. This roughs up the potato surfaces, creating a starchy coating that crisps beautifully in the oven.
Roasting: Spoon the duck fat into a sturdy metal roasting pan (avoid glass or ceramic). Place the pan in the oven and heat for 5 to 7 minutes until the fat is shimmering and very hot, with faint wisps of smoke.
Working quickly and carefully, remove the pan from the oven. Add the potatoes (take care—the fat will be extremely hot), turn them to coat evenly, then arrange them cut-side down. Roast for 25 minutes. Turn the potatoes, roast for another 15 minutes, then turn once more and roast for a further 10 to 15 minutes, until deeply golden, crisp on the outside, and fluffy inside.
Transfer the potatoes to a warm serving bowl. Finish with sea salt flakes and chopped rosemary if using, and serve immediately while irresistibly crunchy.