Steps:

1. 1 Heat oven to 375° F.
2. 2 Using a mortar and pestle grind the garlic, rosemary, thyme and peppercorns into a thick paste. If you don't have a mortar and pestle you can use a small food processor to pulse these ingredients together until a thick paste is achieved.
3. 3 Trim any excess fat from the pork with a small knife and pat it dry with a paper towel. Rub the garlic-herb paste on all sides of the pork and then generously season with salt.
4. 4 In an oven proof skillet or braiser, heat 2 tablespoons olive oil over high until shimmering. Sear the tenderloin until browned on all sides, about 3 minutes per side (12 minutes total). Transfer the skillet to the center rack in the oven and cook until the internal temperature of the pork reads 140°F with a meat thermometer, about 15-20 minutes. Transfer the pork from the skillet onto a cutting board and tent loosely with foil.
5. 5 Set the skillet, over medium high heat, add 1 tablespoon olive oil and sauté the shallots until just tender.
6. 6 Pour the port wine into skillet along with the peppercorns and rosemary. Bring the liquid to a low boil and allow it to reduce to about half while scrapping up brown bits with a wooden spoon. Once the liquid has reduced, turn off the heat and whisk in the butter.
7. 7 Strain the sauce and allow to cool. The port wine sauce can be kept for up to 36 hours and is fantastic as a gravy on mashed potatoes.
8. 8 Thinly slice the pork roast and serve with port wine sauce on the side. Serve over mashed potatoes.