Put the beef sirloin into a heat-resistant dish and spread two tablespoonfuls of soft butter over it. Roast in an oven heated up to 180 degrees for 10–15 minutes until golden brown. Take the sirloin out and leave it to cool down. Pour all juices out of the baking dish.
Melt two tablespoonfuls of butter on a frying pan. Add the chopped onion and the mushrooms cut into cubes; fry for 5 minutes.
Mix the foie gras with two tablespoonfuls of soft butter; season with salt and pepper. Spread the mixture of the foie gras and butter over the cooled beef. Sprinkle with the onion and mushrooms evenly.
Roll the puff pastry until it is large enough to surround the whole sirloin. Put the beef on the dough on one side in the middle. Lay the dough around the sirloin and tie with a rope. Seal all edges, making sure that the seams are not too thick. If necessary, trim the edges of the dough and put them under the beef.
Put the beef in a roasting pan and spread the egg yolk over the beef. Roast at 190 degrees for 10 minutes. Then reduce the temperature to 175 degrees and roast for 10–15 minutes until the dough becomes golden brown.