Preheat oven to 450°. Melt ½ Tbsp. butter in heavy, large oven-proof skillet over medium-high heat. Sprinkle pork with salt and pepper. Sear pork on all sides (about 2 minutes). Place skillet with pork in oven. Roast pork until thermometer inserted in center reads 155°, about 10 minutes.

Melt remaining 1 Tbsp. butter in heavy medium skillet over medium-high heat. Add onion and rosemary. Sauté until onion softens, about 3 minutes. Add broth, cranberry sauce and vinegar. Whisk until cranberry sauce melts, about 2 minutes.

Transfer pork to work surface. Scrape any juices from skillet into cranberry mixture. Boil until sauce has reduced enough to coat spoon thickly, about 6 minutes. Season with salt and pepper. Slice pork and serve.