Wash the potatoes and cook covered for about 20 minutes. Peel and finely dice the onions. Clean, wash and slice the mushrooms. Wash and chop parsley
Whisk the egg yolks. Quench and peel the potatoes. Press hot through a potato press. Season with salt and nutmeg. Stir in egg yolk, up to approx. 1 tbsp, and truffle butter briefly. Pour the puree into a piping bag with a large star-shaped spout and squirt onto the edge of a greased ovenproof dish
Dab the fillets dry. Cut into 4 thick medallions each, press them slightly flat. Fry them in hot oil in portions on each side. Season with salt and pepper, remove and place in the middle of the dish
Heat 2 tablespoons of butter in the frying fat. Fry the mushrooms and onions. Season with salt and pepper. Deglaze with cream, boil down a little. Stir in parsley. Spread mushroom cream on the medallions. Mix the rest of the egg yolk and 1 teaspoon of water, brush the purée rim with it
Spread the crème fraiche as small blobs on the meat. Bake in a preheated oven (electric cooker: 225 °C/recirculating air: 200 °C/ gas: level 4) for 15-20 minutes until golden brown
Drink: dry red wine