Rinse the goose and pat dry. Season the goose with salt and pepper inside and out.
Peel and rinse the potatoes and cook for 10 minutes in salted water. Rinse and peel the carrot and cut into pieces. Rinse the marjoram. Peel the onions and cut into quarters.
Drain the potatoes and mix with the carrots and marjoram and place inside the goose. Secure the opening with wooden skewers or sew opening shut with kitchen twine.
Place the goose, breast side up, in a large roasting pan. Bake in a preheated oven at 180°C (approximatley 350°F) for 3-3 ½ hours. After about 45 minutes skim the fat and place the onions around the goose. Carefully pour about ¾ liter (approximately 1 quart) of water into the pan. Baste the goose with the pan juices throughout cooking time.
Remove the goose from the pan and keep warm. Pour the pan juices through a sieve, pour into a saucepan and skim the fat. Bring the juices to a boil and whisk in the cornstarch to thicken. Season with salt and pepper.