Wash the potatoes and cook covered for about 20 minutes. Quench and peel. Leave to cool for at least 4 hours, preferably overnight
Peel and chop the onions. Clean, wash and slice the mushrooms. Cut cucumbers into strips. Roughly grate potatoes
Heat 2 tablespoons of butter in a large ovenproof pan (28 cm Ø). Add the grated potatoes. Season with salt, pepper and nutmeg. Press everything together to a potato pancake and fry briefly on the bottom. Spread 3-5 tbsp cream and 1 tbsp butter in flakes on top. Then bake in the preheated oven (electric cooker: 225 °C/circulating air: 200 °C/gas: level 4) for about 25 minutes
Dab fillet dry and cut into 12-16 slices. Heat 2 tablespoons of oil in a large frying pan. Fry the meat for 1-2 minutes on each side. Season with salt and pepper and remove
Heat 2 tablespoons of oil in the frying fat. Fry the mushrooms in it. Sauté onions briefly. Season with salt and pepper. Dust with flour and sauté briefly. Stir in 1/2 l water, 150 g cream and stock, bring to the boil. Simmer everything for about 5 minutes. Stir cucumber into the sauce. Season to taste with mustard, cucumber water, salt, pepper and sugar. Heat meat in the sauce
Wash the parsley and chop finely, except for a little bit. Slide Rösti out of the pan and cut into portions. Serve everything. Sprinkle with parsley and garnish