Wash the potatoes and cook covered with water for about 20 minutes. In the meantime peel and finely dice the onions. Clean the mushrooms, clean them and cut them into fine slices. Wash parsley, shake dry and chop, except for something to garnish. Wash the fillets, dab dry and cut into 4 thick medallions each. Press the medallions slightly flat.
Heat the oil in a pan and fry the fillets thoroughly, turning them over. (Beef medallions approx. 4 minutes, pork medallions approx. 3 minutes.) Season fillets with salt and pepper, remove from the pan and place in the middle of an ovenproof dish. Add fat to the hot oil. Fry the mushrooms and onions in it, turning them golden brown. Season with salt, pepper and nutmeg. Remove from the heat. Stir in 100 g of crème fraîche and the parsley, except for a little bit for sprinkling. Spread the mushroom mixture on the fillets as small bonnets/croissants. Drain the potatoes, rinse with cold water and peel. Press the potatoes hot through a potato press and season with salt and nutmeg. Mix the egg yolks and take off about 1 tablespoon. Stir the remaining egg yolk briefly into the hot potato mixture.
Spread the mushroom mixture on the fillets as small bonnets/croissants. Drain the potatoes, rinse with cold water and peel. Press the potatoes hot through a potato press and season with salt and nutmeg. Mix the egg yolks and take off about 1 tablespoon. Stir the remaining egg yolk briefly into the hot potato mixture. Put the potato mixture into a piping bag with a large star-shaped spout and squirt a potato border around the fillets. Mix the remaining egg yolk and 1 teaspoon of water and brush the potato rim with it. Spread truffle butter as small flakes on the edge. Bake in the preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: level 4) for approx. 15 minutes. Put the rest of the crème fraîche as small blobs on each mushroom crust and bake for another 3-5 minutes. Sprinkle with parsley and garnish
Put the potato mixture into a piping bag with a large star-shaped spout and squirt a potato border around the fillets. Mix the remaining egg yolk and 1 teaspoon of water and brush the potato rim with it. Spread truffle butter as small flakes on the edge. Bake in the preheated oven (electric cooker: 225 °C/ convection oven: 200 °C/ gas: level 4) for approx. 15 minutes. Put the rest of the crème fraîche as small blobs on each mushroom crust and bake for another 3-5 minutes. Sprinkle with parsley and garnish