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1
Preparation: You will need an ovenproof skillet large enough to hold the whole tenderloin, or you can cut it in half or use a heavy-duty roasting pan that will fit over two burners and can take direct high heat.
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2
Marinate beef: Trim the beef tenderloin of any fat. Place the tenderloin in a sealable plastic bag. In a glass measure, whisk together the vinegar, 1/3 cup olive oil, garlic, soy sauce and rosemary. Pour the marinade over the beef and seal the bag. Marinate at least 1 hour and up to 3 hours.
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3
Sear, roast tenderloin: Preheat the oven to 400 degrees. Heat the 2 tablespoons of olive oil in a large, ovenproof skillet over high heat. Add the marinated tenderloin, discarding marinade, and sear about 3 minutes on all sides until browned and crusty. Transfer the skillet to the oven and roast about 20 minutes or until desired degree of doneness. (If the skillet or pan becomes too dry, add a little beef broth or water.) For medium-rare, the internal temperature should register about 120 to 125 on an instant-read thermometer.
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4
Let rest: Remove the roast from the oven and transfer to a platter. Tent with foil, and let it rest 10 to 20 minutes before slicing.
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5
Prepare sauce: In the skillet the tenderloin was roasted in, add the beef broth. Heat over high heat, scraping up any browned bits on the bottom of the skillet. Whisk in the balsamic vinegar and continue heating. Whisk in the butter until the sauce thickens slightly.
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6
Serve: Slice the tenderloin and serve drizzled with sauce.
