Preheat oven to 450°F.
Melt 1/2 tablespoon butter in heavy large ovenproof skillet over medium-high heat. Sprinkle pork with salt and pepper. Sear pork on all sides. Transfer to an oven-proof plate or dish and roast until a thermometer inserted into the center registers 155 to 160 degrees F, about 15 to 20 minutes.
Meanwhile, melt remaining 1 tablespoon butter in the skillet over medium-high heat. Add onion and rosemary; sauté until onion softens and begins to pick up color, about 10 to 15 minutes. Add broth, cranberry sauce and vinegar and whisk until cranberry sauce melts, about 2 minutes. Bring to a boil and cook until the sauce has reduced enough to coat the spoon thickly, about 6 minutes. Season with salt and lots of pepper.
When pork is finished, transfer pork to work surface. Scrape any juices from baking pan as well as any juices given off by the resting tenderloin to the cranberry mixture. Slice pork and serve with sauce.
Keywords: Bargain Meal of the Week, Bon Appetit, cranberries, cranberry sauce, leftover cranberry sauce, Pork, Pork Tenderloin