Pork Tenderloin With Balsamic-Cranberry Sauce

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, about 2 minutes.
Place skillet with pork in oven.
Roast pork until thermometer inserted into center registers 155°F, about 10 minutes.
Meanwhile, melt remaining 1 tablespoon 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 and whisk until cranberry sauce melts, about 2 minutes.
Transfer pork to work surface.
Scrape any juices from large 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 with sauce.