Fillet of Beef with Bone Marrow & Parmesan Crostini & Red Wine Balsamic Vinegar Jus

This dish evolved from the French classic, steak Bordelaise, which is beef in a red wine sauce with bone marrow. In my adaptation the marrow is topped with shaved Parmesan and the sauce enriched with aged balsamic vinegar. I generally serve it with buttered spinach with porcini mushrooms and pan-roasted shallots alongside, but any green vegetable would be good.