Dried porcini mushrooms, ground to a fine powder, form the base of the flavorful spice rub for tenderloin, the tenderest cut of beef. To further the savory flavor, we also made a rich mushroom sauce spiked with beef broth and red miso, all of it balanced with white wine and butter. You'll need a spice or coffee grinder to pulverize the dried porcini and the fennel seed. The roast should be trimmed of all visible fat and silver skin before being tied and coated with the rub. The best way to test for doneness of the tenderloin is to use an instant thermometer inserted at the thickest part of the roast.