Wash the duck and offal and pat dry. Rub the duck inside and out with salt and pepper. Clean the pineapple and cut it into slices. Cut off the skin and cut out the hard inner stalk in a circle.
Put two thin slices aside for garnishing. Cut the rest of the pineapple into small pieces. Clean, wash and chop the spring onions and mix with 2/3 of the pineapple pieces. Fill the duck with it, close the opening with wooden skewers.
Place the duck on the oven rack. Slide the fat pan underneath and bake in the preheated oven (electric: 200 ° C / gas: level 3) for 2 1/2-3 hours. Put coarsely chopped offal into the fat pan after 1 hour roasting time.
Spread the duck several times with the roast stock that has formed. During the last 15 minutes, place the thin slices of pineapple on the duck. Peel and finely dice the shallots and sauté in hot clarified butter.
Add the red cabbage. Cover and simmer for about 20 minutes. Add the pineapple pieces after 10 minutes. In the meantime boil up 3/8 litres of water and 1/2 teaspoon of salt for the duchess potatoes. Add milk, remove from heat and stir in the flake puree.
Leave to swell for 1 minute, stir again, allow to cool slightly. Pour the puree into a piping bag with star-shaped spout. Spray small towers onto a baking tray lined with baking paper. Whisk egg yolk and cream and spread on the duchess potatoes.
Remove the duck, wrap in aluminium foil and keep warm for a short time. Bake the potatoes in a hot oven until golden brown. Meanwhile, dissolve the roast with hot broth, pour through a sieve into a small pot.
Skim off the resulting frying fat. Boil up the sauce and thicken with sauce thickener. Season to taste with salt, pepper and sherry. Garnish duck with herbs and arrange on a preheated plate with red cabbage and duchess potatoes.