Rinse goose, and pat dry, season inside and out with salt and pepper and sprinkle with mugwort.
Rinse and halve apples, core and cut into slices, mix with walnuts and thyme leaves and stuff goose with the mixture. Sew the opening well.
Place goose, breast side down, on a greased roasting pan and add 250 ml (approximately 1 cup) of water, wine and apple juice to the pan. Roast in preheated oven at 220°C (approximately 425°F) for about 45 minutes. Reduce oven temperature to 170°C (approximately 350°F) and pierce goose under wings with a toothpick. Roast goose for another 2 hours and turn goose over once. Baste goose often with cooking juices often. Baste goose 15 minutes before the end of cooking with remaining wine and increase oven temperature to 220°C (approximately 425°F) again.
Remove goose from the pan and strain cooking sauce through a sieve, skim fat. Add chicken stock to make 500 ml (approximately 2 cups) of sauce and thicken with a little cornstarch mixed with a bit of water, season with salt and pepper. Whisk sauce until smooth.
Arrange goose on a platter and serve accompanied by sauce. If desired, serve with potato dumplings and Brussels sprouts.