Season the Prime Rib Roast roast generously with salt, pepper, minced garlic, and chopped rosemary. Slow roast in a 275 degree oven until it reaches an internal temperature of 110 degrees for rare, 120 degrees for medium rare, or 130 degrees for medium. Remove roast from oven, cover with foil, and let rest 30 minutes, or until temperature stops rising. While roast is resting, sauté onions in just a little oil, and then deglaze with all the red wine. Add the sprig of rosemary to the red wine while it reduces, and reduce by half. Once the red wine is reduced, pull out the sprig of rosemary, pull it off the heat, and whisk in the cold butter a little at a time. Taste sauce and adjust seasoning accordingly. Slice Prime Rib Roast and serve with the reduced red wine sauce.