Preheat the oven to 200C. Peel the potatoes, cut any large ones in half and parboil.
In a large roasting tray add the goose fat and place the tray in the oven for the fat to get really hot. When the tray starts to smoke slightly, carefully add the potatoes to the fat. Let this cook in the fat for about 3-4 minutes with a turn. Make sure the potatoes are golden and crisp all over and coated evenly in the fat.
Quarter the onions whilst you wait for the potatoes and when ready, add in the onions and rosemary sprigs.
Top the tray up with the cider and the stock and place back in the oven for 40 minutes, turning half way to make sure they aren’t too soggy. Cook until soft and fluffy inside and golden on the outside, serving with any extra sauce for gravy.
