If your tenderloin roast is not trimmed, you will want to trim off any excess fat. Next, you will wrap the tenderloin so the smaller end flaps tuck in closely, making a more uniform roast. This helps maintain an evenly cooked tenderloin and also presents better on the serving plate. Make a slip knot on end of roast furthest from you, securing the Butcher's twine. Next, pull a length of Butcher's twine towards you, making a large loop. This loop will now be slipped over the furthest end of the roast and placed an inch away from your initial anchor. Continue this at one inch intervals the entire length of the roast. Now, run the Butcher's twine back down the length of the roast under the loops you just placed. Secure at the original anchor.
Preheat oven to 500 degrees.
Rub softened butter over tenderloin. Season with salt, pepper and chopped rosemary. Let rest until room temperature while you pre-heat the oven.
Place roast on a sheet pan in the 500 degree oven for 22 minutes or until 125 degree internal temperature is reached. If you have an in oven meat thermometer, I would suggest using.
Remove from oven and place foil over top, rest for 20 minutes. Slice in thick one inch pieces and serve with port wine sauce.