Preheat oven to 200°C/400°F. Dollop duck fat onto a large, heavy-based roasting tray. Place tray onto the bottom shelf of the oven to get the fat nice and hot. (note 1)
Meanwhile, peel and cut potatoes into large chunks, to about 5cm/1.5inch cubes.
To parboil the potatoes, add potatoes and cold water (enough to cover the potatoes) into a large pot and bring it up to boil. Generously salt the water. Boil potatoes for about 7 minutes, lid off.
Once 7 minutes are up, turn the stove off, drain potatoes well. Allow potatoes to steam to evaporate excess moisture. (note 2)
Now, gently shake the pot (or colander, if using) to fluff up the edges of the potatoes for an extra crunchy exterior.
Carefully, with oven mitts on, take the tray out of the oven and gently place potatoes onto the tray. Using a pair of tongs, turn and coat the potatoes with the duck fat thoroughly.
Space potatoes out, with wiggle room between each other without overlapping. Bake on the bottom shelf for 45 minutes (or until potatoes a deep golden and crispy), turning every 15 minutes. (note 3)
After the first 15 minutes of roasting, scatter lightly crushed garlic cloves (skin on) and rosemary sprigs all over the potatoes before returning it into the oven. (note 4)
Season with salt and pepper. Serve immediately whilst it's hot and crispy!