Boil: Peel and cut all potatoes to roughly the same size. Boil for 15 minutes in salted water. Remove and leave in a large colander so the moisture can steam away. Give them a shake (chuff) to break the edges slightly.
Roast: In a large roasting dish heat the duck fat in the oven at 180°C
Carefully tip the potatoes into the hot fat, add in the garlic and thyme, season and return to the oven. Roast for 30 minutes, turning the potatoes in the fat every 10 minutes.