Place puff pastry sheets next to each other and allow to thaw at room temperature. Peel and finely dice the onions. Clean and finely chop the mushrooms. Heat 2 tablespoons of oil in a frying pan, fry the mushrooms and onions until there is no more liquid in the pan.
Season with salt and pepper and let it cool down. Heat 1 tablespoon of oil in a pan. Fry the fillet of beef for about 10 minutes, turning it over. Season with salt and pepper. Let cool down. Place the puff pastry sheets on top of each other and roll out into a square (approx. 35 x 35 cm) on a lightly floured work surface.
Separate the egg and brush the edges of the dough with egg white. Sprinkle breadcrumbs in width and length of the fillet on the front edge of the dough and add some mushroom sauce. Place the fillet on top. Put the rest of the mushroom sauce on and around the fillet and press on.
Wrap the fillet in the dough, press the sides well together. Cut off excess side edges and roll out again. Cut the dough with a wavy pastry wheel into strips of approx. 1 cm width. Brush with egg white and cover the fillet with it.
Place the fillet on a baking tray lined with baking paper. Mix egg yolk and cream. Spread the dough with it. Bake in the preheated oven (electric cooker: 200 °C/ circulating air 175 °C/ gas: level 3) for approx. 20 minutes (it is best to place a meat thermometer in the middle of the fillet, target temperature 56 °C).
Wash the carrots, peel them and leave some of the green on. Take the fillet out of the oven and let it rest for about 10 minutes. Cook the carrots in salted water for 6-7 minutes. Caramelize sugar and add orange juice.
Simmer the caramel for about 5 minutes. Wash parsley, shake dry and chop leaves of 3 stalks coarsely. Drain the carrots and toss in the caramel. Add chopped parsley and season with salt and pepper.
Cut fillet into slices and arrange with caramel carrots. Garnish with the remaining parsley.