Wash goose breast, pat dry. Season the bone side with salt and pepper, skin only with pepper. Onions peel, quarter. Roast everything in the roaster in a preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: level 3) for about 1 1/2 hours. Gradually add approx. 1/4 l water and half the stock. Top the breast with the stock from time to time
Wash the sage and cut the leaves finely. Peel, wash and coarsely grate the potatoes. Mix with egg, flour and sage, season
Spread goose breast with cold strong salted water for the last 10 minutes and fry until crispy at highest heat. If necessary, spread with orange marmalade after another 5 minutes
Meanwhile, fry approx. 16 golden brown Rösti in hot oil in a coated pan
Keep goose breast warm. Sieve the stock. Dissolve the meat with the remaining stock and add it. Skim off fat. Bring sauce with Marsala to the boil and simmer for a while. Stir starch and 3 tbsp. water until smooth. Thicken sauce with it and season to taste. Remove meat from the bone and cut open. Arrange everything and garnish if necessary. Goes well with butter carrots