Place the beef and broth in a 6-quart Dutch oven, making sure the beef is totally covered by the stock, and cover with foil, leaving a small opening for steam to escape. Poke a small hole in the foil and insert an instant-read thermometer into the meat. Cook over low heat until the beef reaches 125 degrees F, 55 to 60 minutes. Transfer the beef to a cutting board and let stand 10 minutes.
In a medium saucepan, bring the port to a boil and then reduce until about 1/3 cup. Remove from the heat and whisk in the butter. Season with salt and pepper.
To serve, slice the beef, top with sauce and garnish with parsley.
What Makes This Recipe Really Sing: Because the beef is brought to temperature up so slowly, it is very tender. It would make a killer steak sandwich.