The beef tenderloin is seasoned with a little salt and pepper and vacuum sealed. Place the meat in the water bath (sous vide) for 2 hours at 51 degrees Celsius (124 degrees Fahrenheit). The mushrooms should be blended, for example with an immersion blender, until they are pitch black. Quickly sauté the onions and garlic in a pan, then add the mushrooms. Continue cooking for a long time, approximately 30-45 minutes, until all the liquid is gone.  Remove the meat after the 2 hours, pat it dry with a towel/paper towel, and quickly sear it on all sides in a pan with butter, oil, and rosemary. Lay out plastic wrap on the kitchen counter - place 5 slices of ham in a row over it. Spread the mushroom/onion mixture in a very thin layer over the ham. Coat the meat on all sides with mustard and place it on top. Then roll the whole thing into a sausage shape and place it in the freezer for 15 minutes. Remove the sausage from the freezer, discard the plastic wrap, and place it on the puff pastry sheet for wrapping. Score the top of the puff pastry, brush it with egg yolk, and bake it in a convection oven at 225 degrees Celsius (437 degrees Fahrenheit) for 10 minutes. The meat is difficult to cut with a standard knife, so use a bread knife for this.