A whole beef tenderloin not only is quick and easy to roast, it also is dependably tender despite its leanness. But because its flavor is quite mild, we use a trio of bold ingredients—mushrooms, rosemary and black pepper—to bring depth and savoriness. We create a seasoning rub by finely grinding umami-packed dried porcini mushrooms (shiitakes work, too), rosemary and pepper, and we roast the meat over a layer of fresh portobello mushrooms and sliced onion. The mushroom-onion mixture becomes a rich, flavorful accompaniment to the meat. Because portobellos contain a lot of water, we give them a head start on cooking and add the tenderloin to the pan only after they begin to brown. You will need a large roasting pan—one that measures about 12 by 15 inches—along with a V-style roasting rack.