Sprinkle all sides of meat generously with coarse Kosher salt and place in fridge uncovered for 24-36 hours.
When ready to smoke meat, remove from refrigerator to come to room temperature.
preheat smoker + prepare garlic herb rub
Preheat smoker to 225° F according to your smoker's directions.
Combine all garlic herb ingredients in a small bowl and stir until well combined.
If you did NOT dry brine your roast, this is where you would add salt to taste.
Using clean hands, spread the rub evenly on 3 sides of the prime rib roast. Adding rub to the fat cap is optional.
Place beef directly on grill grate FAT SIDE UP and insert your smoker's meat thermometer probe in the thickest part of the roast.
Smoke for 35 minutes per pound (about 5 1/2 hours for a 15 lb roast) or until internal temperature reaches 120° F for rare (with resting time).
Wrap the roast in non-coated butcher paper or aluminum foil and let meat rest for about 30 minutes or until temperature reaches 125° F and juices are all redistributed.
Unwrap meat, place on a cutting board and slice into individual slices. Serve with au jus and fresh horseradish.