Preheat oven to 450°F. In a heavy skillet, heat a drizzle of oil and dab of butter over medium-high heat. Sprinkle pork with salt and pepper and sear on all sides, until golden all over. Place the pork in a small baking dish (or loaf pan) and roast until thermometer inserted into the thickest part registers 155°F - about 15 minutes.
Meanwhile, melt another dab of butter in the skillet; add the onion and sauté until soft and starting to turn golden. Add the broth, cranberry sauce and vinegar and whisk until cranberry sauce melts and the sauce thickens.
Let the pork rest a few minutes before slicing; serve immediately, with the sauce.