Spread the horseradish mayo all over the outside of the prime rib roast
Chop the garlic cloves and chop the rosemary leaves. Then spread them all over the roast as well. Season the outside of the roast with salt and pepper.
Place the prime rib roast into your preheated smoker. Smoke at 250℉ for three hours or until the internal temperature reaches 120℉.
Remove prime rib roast from smoker and place on hot griddle to reverse sear. Sear each side for about 2-3 minutes on the hot griddle. Watch the internal temperature. Remove the roast when the internal temperature reaches 125℉.
Allow the prime rib roast to rest for 20 minutes. The internal temperature will increase to about 130℉ for a perfectly cooked medium rare doneness.