Peel the potatoes and cut into desired size.
Cook in salted water for approx. 10 minutes until edges of the potato begin to break up. Remove from the water and drain through a colander, shake the potatoes in the colander to ruffle the edges. Leave to steam dry.
Preheat the oven to 200C/Fan 180C/Gas 6. Heat the duck fat in a tray and add garlic and rosemary. Once hot, add the potatoes to the tray and coat each piece of potato with duck fat.
Roast in the oven for 15 minutes, turn the potatoes and roast for a further 15 minutes and keep checking until roasted to desired colour.