01 - Set the oven to 425°F and allow it to reach temperature.
02 - Place potato chunks in a large saucepan and cover with cold salted water. Bring to a boil and simmer for 8-10 minutes until just tender but intact.
03 - Drain the potatoes and let them steam dry in the pot for 2 minutes. Gently shake the pot to roughen edges for enhanced crispiness.
04 - Place duck fat in a large roasting pan and heat in the oven for 5 minutes until shimmering and hot.
05 - Carefully add potatoes to the hot duck fat in the roasting pan, turning with tongs to coat evenly. Add garlic and rosemary if desired.
06 - Roast potatoes for 40 minutes, turning once halfway through until golden, crisp, and deeply browned.
07 - Remove from oven and immediately sprinkle with remaining sea salt and black pepper. Serve hot.