Clean, clean and slice the mushrooms. Wash sage, shake dry, pluck leaves from the stems. Peel potatoes and wash them thoroughly. Coarsely grate the Parmesan cheese.
Peel the onions. Grate potatoes coarsely and 1 onion finely. Mix potatoes, grated onion, parmesan, egg and starch well. Season with salt, pepper and nutmeg. Heat 1 1/2 tablespoon of oil in a frying pan, fry the mushrooms in it while turning.
Dice 1 onion, add and fry briefly. Add sage, except for 2-3 leaves for garnishing. Season with salt and pepper. Deglaze with cream and stock, bring to the boil and simmer at low heat for about 10 minutes.
Season again with salt, pepper and sugar. Heat 1 tablespoon of oil in a large pan. Fry 3 Rösti from the potato mixture for about 2 minutes on each side. Remove and keep warm.
Fry another 5 Rösti in 2 portions in 1 tablespoon of oil each in the same way. Skip the bacon in two portions in the pan. Arrange the Rösti and bacon with a little sauce on a plate. Garnish with sage.
Add the rest of the sauce separately.