I liked how he prepared the mushroom mixture. This sandwich is all about "the beef" and how to maximize its flavour. You would want to do it through umami rich foods which he did. The addition of the bone marrow, worcestershire sauce (the fermented anchovy that is present)and brandy reduction all enhance the "meatiness" of the beef really well. That is another reason I would use a mushroom like shiitake as the base instead of white button mushrooms since they are very rich in umami. Adding just a pinch of ground star anise would really kick the mushrooms mixture up to a whole another level as well. Not enough to taste but to maximize the umami that is going on in this sandwich.
