Fry the onions, hearts, and livers in butter, add mushrooms, and flambé with brandy.
Combine the fried mixture with the remaining stuffing ingredients in a bowl.
Preheat the oven to 160°C.
Stuff both ends of the goose and seal with string or skewers.
Place the goose on a trivet in a baking tray.
Cook the goose for 105 minutes.
Baste the goose, increase heat to 200°C, and cook for another 15 minutes.
Boil potatoes in salted water for 8 to 10 minutes until slightly underdone.
Drain potatoes and shake in the colander to roughen the edges.
Remove goose from oven, cover with foil, and rest for 20 to 30 minutes.
Increase oven temperature to 220°C.
Heat goose fat in a baking dish on the hob until smoking hot.
Add potatoes to the fat and roast in the oven for 30 minutes until crispy.
Boil goose juices and mushroom liquid in a pan until reduced into a gravy.
Slice the goose and serve with roast potatoes and gravy.