Place the potatoes in a pan, cover with cold water and bring to the boil. Simmer for 8-10 minutes, or until tender. Drain. Place half the butter in a large frying pan over a low heat. Fry the onions and 2 of the chopped apples for 4-5 minutes, until soft.
Add 2 tbsps wild rosemary, season, then stir in the cooked potatoes. Fill the small neck cavity of the goose with the stuffing. Secure with metal skewers. Place any leftover stuffing in an ovenproof dish to cook separately.
Using the pack instructions, weigh the stuffed goose to calculate the cooking time and preheat the oven accordingly. Place the goose on a metal rack over a large roasting tin and cook. Add the dish of the remaining stuffing to cook for the final 30-40 minutes, or until piping hot and cooked through. Check the goose is thoroughly cooked with no pink meat visible.
For the apple sauce, place the remaining apples, butter, rosemary and the sugar, plus 150ml cold water, in a medium-size pan and simmer for 8-10 minutes, or until the apples are soft. Keep warm until ready to serve. Remove the stuffing from the cavity. Carve the goose and serve with the apple sauce.