Preheat oven to 250F. Place wire rack onto baking sheet.
In bowl, combine miso, butter, pepper and thyme; rub all over rib roast. Place onto prepared baking sheet.
Roast until instant-read thermometer inserted into thickest part of roast reads 130F. (Tip: test at multiple points of roast to be sure) Remove roast from oven. Increase temperature to 550F.
Place roast in oven until well browned. Slice and serve with Prime rib sauce (if desired).
Prime rib sauce:
In bowl, combine all ingredients. Cover and refrigerate for minimum one hour or overnight.