The day before, wash the potatoes for the Rösti and cook them unpeeled in boiling salted water for about 20 minutes. Drain, steam briefly and let cool. Refrigerate overnight.
The next day, wash the pumpkin, cut it in half, remove it and mash it. Sprinkle with a little salt and leave to drain in the colander for 30 minutes. Meanwhile, peel and grate the potatoes. Squeeze the pumpkin well with your hands. Mix potatoes, pumpkin, egg, egg yolk, breadcrumbs, salt and nutmeg.
Wash the mushrooms and cream the mushrooms. Cut the mushrooms into thick slices. Lighten the schnitzel between the foil. Place them next to each other on the work surface, spread very thinly with horseradish, season with pepper. Sprinkle with 1 tablespoon parsley, 1 sage leaf each and a slice of ham. Fold the schnitzel over and secure with toothpicks. Finely chop the onion.
Heat 2-3 tablespoons of oil in a wide pan. Season the meat with salt and pepper and fry briefly on both sides in hot oil over high heat. Remove from the pan, add 1 tablespoon of oil and fry the mushrooms in it for 2 minutes. Add onion, fry briefly, salt and pepper. Fill with stock and cream and let boil until creamy. Add the schnitzel, reduce the heat and let the meat simmer for 5 minutes.
For the rösti, heat the oil in a covered pan. Add the hash browns one tablespoon at a time and fry over medium to high heat for 4-5 minutes. Carefully flip the hash browns and fry for another 3-4 minutes.
Finely chop the best and sprinkle the remaining parsley over the schnitzel. Arrange schnitzel with mushroom cream and rösti on plates.