Seriously, the sauce is what makes this dish. It uses all the good browned bits left from the steak, and then shallots and mustard seeds get sautéed, wine and lemon zest get added and at the very end a large knob of butter for added richness. The mushrooms are golden brown and are served on top of toasted bread. You can add the steak for an open faced sandwich, use the bread to sop up the sauce, or eat everything separately. However you choose to eat this meal, one thing is guaranteed…..there won’t be any leftovers.