The steak should be about 2 inches thick, cut from some fine animal. Cut one end off the bread and remove as much center crumb as necessary to allow the steak and mushrooms to fit in snugly when cooked. Quickly sear the meat in a very hot pan, keeping the meat rare. Take from the fire and season well on both sides, then insert it into the hollowed loaf. Grill enough mushrooms to cover the meat and place them over the steak. Replace the removed crust end of the loaf.
Wrap the whole thing in a double sheet of white blotting paper and secure with string. Then secure again with parchment paper and more string. (Or use foil and film). Place under a weighted board for 6 hours. When eating this sandwich just cut off a slice as required.