Prepare Horseradish Sauce: Mix together sour cream and horseradish, cover and chill in refrigerator for at least 30 minutes. May be prepared the day before.
Prepare the Prime Rib: Rub the roast with olive oil. Mix the minced garlic, salt, black pepper, rosemary, thyme, onion powder, and garlic powder in a bowl. Massage the seasoning mixture all over the prime rib, making sure it's evenly coated. Wrap the roast in plastic wrap and let it sit in the fridge for at least 4 hours, preferably overnight.
Preheat the Smoker: Preheat your Masterbuilt Gravity 800 smoker to 225°F. Add hickory or oak wood chips to the hopper for a rich, smoky flavor.
Smoke the Prime Rib: Place the seasoned prime rib directly on the grill grates, bone-side down. Close the lid and smoke for approximately 4 hours, or until the internal temperature reaches 130°F for medium-rare (adjust cooking time based on your preferred doneness). Allow for 35 minutes per pound of cook time.
Rest the Meat: Once the prime rib reaches the desired internal temperature, remove it from the smoker, wrap it loosely with aluminum foil, and let it rest for 30 minutes. This allows the juices to redistribute throughout the meat.
Carve and Serve: After resting, carve the prime rib into slices and serve hot. Enjoy the rich, smoky flavor and tender texture of your smoked prime rib!