Pork Tenderloin with Balsamic Cranberry Sauce

2-3 lb pork roast tenderloin
6 tablespoons butter - divided
1/2 onion - finely chopped
3 tablespoons chopped fresh rosemary
1/2 can chicken broth (7 oz)
1 can whole berry cranberry sauce
3 tablespoon balsamic vinegar

Directions
1. Melt 2 Tbs butter in heavy large ovenproof skillet over medium-high heat.
2. Sprinkle pork with salt and pepper.
3. Sear pork on all sides, about 2 minutes.
4. Place pork in crock pot and cook on high with 2 cups of water for 1 1/2-2 hrs.
5. Meanwhile, melt remaining 4 tablespoon butter in heavy medium skillet over medium-high heat.
6. Add onion and rosemary; sauté until onion softens, about 3 minutes.
7. Add broth, cranberry sauce and vinegar and whisk until cranberry sauce melts, about 2 minutes.
8. Boil until sauce has reduced enough to coat spoon thickly, about 6 minutes (could take longer).
9. Season with salt and pepper.
10. Slice pork and serve with sauce.