You could make that sandwich much better by using a less fatty and more tender cut of beef to bite into such as the filet. Filet would be easier to chew cold. I like fat on meat as much as the next but I don't like eating cold fat. Rather than just using white button mushrooms using a medley of mushrooms like portobello, shiitake and chanterelles which will enhance the flavour of the beef more. White button mushrooms are mild and have no flavour. Love the idea of using bone marrow (my favorite food in the world) to help sear the mushrooms. Foie Gras would work great as well.
