As given away by its name, the tenderloin is the most tender cut of beef. The famous filet mignon is a steak taken from this cut. Yet despite its reputation as the king of steaks, the tenderloin is noticeably lacking in flavor. Marinate it or give it a generous coat of seasoning to impart some flavor, then seal it in with a nicely-browned, crisp exterior by pan-searing it. Avoid overlooking the steak by finishing it in the oven.
Marinate the tenderloin steak in the refrigerator, if desired, for 1 to 3 hours. Use equal parts cooking oil and an acidic component, such as barbecue sauce, soy sauce, wine or fruit juice. Add in some salt, pepper and any other herbs and spices you choose. Soak the meat in a glass dish or sealed plastic bag, flipping it over once halfway through. Remove the steak from the marinade and wipe off the excess about 45 minutes before cooking it. Dispose of the marinade or, if you want to use it as a sauce, bring it to a boil for a full 5 minutes to kill bacteria from the raw beef.
Add salt and pepper liberally to all the steak's surfaces. Set the oven to 375 degrees Fahrenheit about 20 minutes before cooking.
Get a heavy-bottomed oven-safe skillet nice and hot over high heat for a few minutes. Cast-iron pans are the best option. Pour in a considerable coating of cooking oil, roll it around and let it heat up.
Place the tenderloin in the pan and leave it in place until it develops a crust, about one to 2 minutes, then turn it with tongs or a spatula. Sear the other side in the same manner.
Insert an oven-safe meat thermometer into the center of the steak. Put the tenderloin directly into the middle of the oven. Cook it to 125 F, then promptly remove it and transfer it from the pan to a plate. Cooking time differs based on several factors, most significantly the thickness of the steak and accuracy of your oven temperature. As a guide, expect a 1-inch steak to take about 8 minutes, minus the total searing time, to reach 125 F; add 2 minutes for 1 1/4-inch-thick steaks, then add about 1 minute for each additional 1/4 inch of thickness.
Rest the beef for about 10 minutes. While resting, the steak's internal temperature rises another 5 to 10 degrees, bringing it right to 130 F to 135 F, the medium-rare window. Resting also provides opportunity for the beef's juices to be reabsorbed back into the muscle fibers so they don't bleed out when you cut into your dinner.