Preheat oven to 500F.
Combine the salt, pepper, and garlic powder in a zip closure bag.
Very lightly oil the tenderloin with olive oil.
Sprinkle the mixture liberally over all sides of the tenderloin and press into the meat.
Heat 1 tablespoon olive oil in a heavy ovenproof skillet or roasting pan over medium high heat.
Sear the tenderloin for a minute or so on all sides.
Add 1/4 cup of beef broth to the skillet (to prevent smoking in the oven and keep meat moist) and the sprig of rosemary.
Transfer skillet to preheated oven and roast tenderloin for about 8 - 10 minutes.
Remove skillet from oven; transfer tenderloin to a cutting board, cover with aluminum foil and allow meat to rest. You can prepare up to this point and hold the meat under the foil for at least 30 - 45 minutes.
Do not wash skillet or discard the roasting juices and bits. With skillet still hot, add the chopped shallot and cook for a minute or so. Add ½ cup beef broth to the skillet, scraping all the caramelized juices and bits with a spoon.
Add the Cabernet Sauvignon and bring to a boil to reduce the volume of the sauce by about half.
If holding the final preparation, stop at this point, cover skillet, and turn heat off.
Just before serving, slice the tenderloin into 3/4" to 1" medallions.
Return skillet to medium high heat, add about 2 tablespoons unsalted butter to the sauce and allow it to melt, taste and if needed adjust with a pinch or salt or pepper. Remove the rosemary sprig.
Return the medallions to the hot pan and gently warm them in the sauce for 30 seconds to 1 minute on each side for medium rare or a bit longer until desired temperature.
Remove the medallions to warmed platter or serving plate, pour sauce over them, and serve.