Open the flame broiler on your Grill and start on 'smoke' with the lid open until a fire is established in the burn pot (3-7 minutes). Preheat to HIGH.
When the grill has preheated, sear the outside of the roast over the open flame until all angles are dark brown. Close the flame broiler once finished.
Mince the garlic and sage. Combine with the rosemary sprigs, salt and pepper. Drizzle olive oil over the roast and sprinkle on the rub mix, covering the whole roast.
Roast on HIGH for about 30 minutes. Reduce the heat to 300F and continue to roast until your Meat Thermometer reads 130F (another 25-30 minutes). Remove from your and rest for 25 minutes. Slice and serve warm.