1) Put a roasting pan (big enough to hold all of the potatoes in a single layer) into the oven and heat at 200°C (fan oven 180°, gas mark 6).
2) Peel the potatoes and cut each one into 4 even size pieces. Put them in a pan and add enough water to just cover. Add salt and wait for the water to boil.
3) As soon as the water comes to a full boil lower the heat and simmer the potatoes for 5-7 minutes or until the potato starts to break apart around the edges.
4) Add the duck or goose fat to the roasting pan along with the rosemary and put it back in the oven for a few minutes, the fat should be extremely hot.
5) Drain the potatoes in a colander and shake, tossing the potatoes about to fluff them up.
6) Place the potatoes into the pan of hot fat, rolling them to coat them evenly. They should sizzle when you add them to the fat.
7) Roast the potatoes in the oven for 15 minutes, then turn potatoes and cook for another 15 minutes. Turn once more and cook for a further 10- 20 minutes or until the potatoes are golden and crispy.
8) Sprinkle with sea salt and cracked black pepper and serve hot.