– Combine salt, pepper, and garlic powder in a small bowl. Set aside. – Pat dry prime rib roast thoroughly with paper towels. Generously coat with salt mixture evenly on all sides, pressing to adhere. Place roast on a plate, cover, and chill in fridge
– Remove roast from fridge about 6-8 hours before serving time and allow it to come to room temperature.
– Set oven to 200F. Insert meat thermometer probe into center of roast and place roast on middle rack of oven. Cook until internal temperature is 10 degrees lower than desired doneness. For medium rare (130F), remove from oven when internal temperature reaches 120F.
– Remove roast from oven, loosely tent with foil, and allow roast to rest at least 30 minutes. Do not remove thermometer probe, nor slice into the roast at all, or juices will escape.
– In a saucepan set over medium-high heat, melt the butter. Add onion and stir 2 min. Add garlic and stir 1 min. Add thyme, rosemary, bouillon, black pepper, and broth. Stir well to combine. – Bring to a boil and continue to boil until liquid is reduced by half of its original volume. – Reduce heat to a low boil, and add a little bit of the dissolved cornstarch at a time, stirring constantly – use only as much as is needed for sauce to reach desired thickness. Remove sprigs of herbs. If desired, au jus can be drained for a smooth sauce. – Just before serving, set oven to broil and brown all sides of roast, rotating as needed. Slice and serve with warm au jus.