Set immersion cooker in a water tub and set temperature at 135°F to preheat.
In a small bowl combine the rosemary, salt, pepper, onion powder, and garlic powder.
Cover roast with olive oil and rub dry ingredients onto roast on all sides.
Place roast inside a zip top freezer bag (or Stasher bag). Add the Worcestershire and beef broth into the bag and seal. Pro tip: Double bag the roast in case the inside bag leaks.
Insert bag into preheated water and sous vide for 6 hours*.
Remove bag from sous vide container and remove roast. Internal temperature of roast should be 135°F using a thermometer. If not, return to sous vide for 30 minutes and check temperature.
Preheat the oven to 450°F on broil.
Place finished roast onto a rack in a roasting pan, then put into the oven for 5-10 minutes, or until dark brown in color.
Remove roast from oven, and place roast on cutting board and allow to rest for 10 minutes.
Meanwhile, strain au jus liquid from the bag through a strainer and add to same skillet, along with beef stock and beef base. Simmer for 5-10 minutes, reducing liquid by half for the au jus.
Stand bones upright and cut the bone off the roast. Place the cut side directly down on the cutting board and slice to desired thickness. Serve with au jus and horseradish.
Notes
*Rib roast should reach 135°F by 6 hours. It can be left in the sous vide for to 8 or 10 hours, as the temperature will remain as set on the immersion cooker and it will not overcook.*Fresh or previously frozen roast can be used, just make sure your roast is fully thawed before cooking!