Preheat your grill to 225-235°F.
While the grill is preheating, prepare your prime rib by applying a light coat of olive oil over the entire prime rib. Using a sharp knife, make several evenly spaced apart incisions (about one inch deep and one inch wide) around the prime rib.
In each incision, push in a half of fresh garlic clove. Season entire roast with fresh rosemary as well as salt and pepper.
Insert a reliable meat thermometer in the roast, place roast in cast iron pan (or any oven safe pan) in order to collect the drippings and put the pan on the grill over indirect heat.
Spoon juices collected in pan over roast about every 20 minutes. When internal temperature reaches 125°F, remove roast from grill and increase direct heat to high.
Place roast directly over high heat for a total of 5 minutes, turning often so that all sides get a nice crust built up.
Remove roast from heat, place on cutting board, and tent with foil. Allow to rest for 10-15 minutes before slicing.
Using a sharp knife, slice roast in ½ inch slices and serve.