The tenderloin is part of the psoas muscle of the cow that rests in the backbone, just between the sirloin and the rib. Because it's not an active muscle, the meat has a tender texture and mild flavor. Steaks cut from the tenderloin are called "filet mignon," French for "dainty" or "cute" fillet.
A classic filet mignon measures just 1-inch in diameter, because it should be cut from the tail, or smallest, end of the tenderloin. In most markets and restaurants, though, steaks cut from the tenderloin that measure 1- to 2-inches thick and 2- to 3-inches in diameter still bear the name "filet mignon." You may also find these thicker, larger cuts labels as "tenderloin steak." Tournedos and medallions are other names for filet mignon.
The term "beef tenderloin" refers to the whole muscle, which weighs, on average, 5 pounds. Tenderloin cooked in smaller sections that serve two or more people may be called Chauteaubriand or Filet de Boeuf. The tenderloin's tenderness makes it a sought-after cut, but it doesn't offer up a lot of flavor.
A classic preparation for a whole beef tenderloin involves crusting it in herbs and spices, searing it in hot oil and then roasting it in a 375-degree Fahrenheit oven. Filet mignon are often wrapped in bacon and seasoned liberally before being seared over high heat and finished in the oven. The mild-flavored steaks pair well with classic French sauces, such as Bordelaise or Bearnaise.
Which to Choose?
If you're hosting a large dinner party, roasting a whole tenderloin is simpler than preparing individual filet mignons. The tenderloin makes a dramatic presentation that's easy to carve and, because of its juiciness, requires no sauce. A low fat content means filet mignon cook quickly and are prone to drying out. It's best to reserve these for small intimate dinners so you can monitor the steaks carefully and avoid overcooking.