Preheat oven to 425 degrees.

Generously season pork tenderloin with salt and pepper, be really generous.

Heat 2 tablespoons olive oil and 2 tablespoons butter in a heavy, oven-proof skillet over medium-high heat. Sear tenderloin on all sides, using tongs to move it around. If you don't have an oven proof skillet, just use a regular skillet, then transfer to an oven safe dish.

Remove pan from heat and place in the oven for 10 to 15 minutes, or until thermometer inserted registers 155 to 160 degrees. (Since I used a pork loin roast  that was a little icy it took more like 30-40 minutes to cook mine to 155 degrees) Set pork on a cutting board to rest (I let it rest for about 5 minutes).

In a separate skillet while the pork loin cooks, heat 1 tablespoon olive oil and 1 tablespoon butter over medium heat. Add onion and cook for a minute or so until slightly tender and chicken broth. Add rosemary. Stir together, then add cranberry sauce.

Heat mixture over medium heat and cook for two or three minutes. Add more cranberry sauce to achieve desired consistency.

Check seasoning and add more salt or pepper if needed (I did taste the sauce and added some salt and pepper).