What is the best way to cook a 15 1/2 pound semi boned (boned and tied) prime rib roast?

Low and slow! Sear it a bit first then into oven at 250 until it reaches temp. This method is foolproof!

Make a dry spice rub of garlic powder, onion powder, paprika, fennel seed, tarragon, black pepper and salt. Rub the prime rib thoroughly with mixture and roast in the oven at 275 degrees for 3-4 hours or until the center measured with a kitchen thermometer reads
120 degrees for a perfect medium rare.