Preheat the oven to 200°C.
Place the leek and onion into the goose cavity.
Prick the goose skin and rub with salt and pepper.
Rub fat over the legs and cover them with foil.
Line a deep roasting dish with enough foil to enclose the bird.
Rest the goose on a rack in the tin, cover with foil, and roast for 210 minutes.
Uncover after one hour, baste regularly, and roast uncovered for the final 45 minutes.
Roast par-boiled potatoes in the drained goose fat for 45 minutes.
Boil giblets in 2.5 litres of water for three hours, strain, and thicken with cornflour.
Soak prunes in port for two hours.
Sauté onion, celery, and garlic in butter until softened.
Chop prunes and sauté with chestnuts and cranberries for one minute.
Add port, simmer until liquid reduces, and stir in parsley before serving.
Carve the goose and serve with potatoes, stuffing, and gravy.