Preheat the oven to 180°C.
Place potatoes in a saucepan of cold salted water and bring to the boil. Reduce the heat to medium and cook for 5 mins or until par-boiled. This leads to crispy edges and fluffy interiors.
Drain, then return to the pan and move the pan around well over medium heat to dry out the potatoes and create rough edges (this will help them get crispy).
Spoon the duck fat into a roasting pan and place in oven for 5 mins to cook down.
Remove from the oven, carefully add the potatoes and (watch for splashes) gently toss to coat evenly.
Season with salt and pepper. You can also use rosemary, thyme or any of your preferred seasonings.
Return to the oven and roast, turning twice for 45 mins, or until the potatoes are golden and crisp (every oven is different so be sure to test a few). Sprinkle with flake salt and serve.