Remove roast from the fridge 2 hours before you start cooking. You want it to come closer to room temperature to not shock and tighten up the meat when cooking. 

In a small bowl combine butter, rosemary, garlic, salt and pepper until well blended.

Rub the butter evenly over the entire prime rib. 

To Sous Vide Cook: Place the rubbed meat into a bag and vacuum seal. Place in water, with the sous vide set to 130 degrees for medium rare. Let cook for 6 hours, you can leave it in up to 10 hours. Remove from heat and sear all sides of the meat in a hot pan. 

To Bake: Preheat oven to 500 degrees. Place the meat, fat side up in a roasting pan. Place in oven, and bake for 5 minutes per pound. After the cooking time is up, turn off the oven. DO NOT OPEN THE DOOR. Let the meat sit in there for 2 hours for medium rare. Let sit longer for more done. Remove from oven and let rest for 10 minutes before slicing.

To make the Au Jus: Pour the juice from the pan (or bag if cooking via sous vide) into a skillet.  Heat over medium heat.  Whisk in flour to make a roux.  Add beef stock and bring to a simmer. Cook for 10 minutes and serve.