The chateaubriand, a thick steak cut from the beef tenderloin, makes an elegant main course for a dinner for two. Here, it is coated with a simple rub of dried shiitake mushrooms before it is briefly roasted in a hot oven. The flavors in the rub are echoed in a sauce of fresh shiitakes and onions. If you wish, substitute fresh cremini or button mushrooms for the fresh shiitakes. Just a drizzle of white truffle oil provides a luxurious accent to the finished dish. Marsala, a fortified wine from Sicily, has a rich, almost smoky character that complements the earthy mushrooms.