In a small bowl, mix together the brown sugar, garlic powder, onion powder, paprika, black pepper, salt, cumin, cayenne and white pepper. Using your hands, coat the meat in the olive oil, then smear the rub all over. Transfer the meat to a plate, cover with plastic wrap and chill in the fridge for at least 1 hour, or up to 4 hours.
Fill a roasting pan (preferably a disposable aluminum pan) by two-thirds with water and add the garlic cloves and onion. Place the pan under the grill rack of a gas grill over the center burners. Light the outer burners, leaving the middle burner unlit, and preheat the grill to 225 degrees F. Remove the meat from the fridge 30 minutes prior to cooking, to allow it to come to room temperature.
Place the meat on the grill rack above the pan (you want to make sure the drippings from the meat are caught by the pan). Close the lid and cook, checking the pan occasionally and adding more water if necessary, until the meat reaches an internal temperature of 180 degrees F, 7 1/2 to 10 hours total, or 1 1/2 to 2 hours per pound. (This sounds like a high internal temperature, but it is necessary for the connective tissue to break down, resulting in fork-tender meat.) Remove the meat from the grill, cover with foil and let rest for 10 minutes; dispose of the drippings.
Slice the meat off the bone and serve immediately.
If you have a smoker or smoking function on your grill that would work really well with this cut of meat. Hickory, oak or walnut chips add great flavor. Smoke the meat until the internal temperature reaches 180 degrees F.
A viewer or guest of the show, who may not be a professional cook, provided this recipe. It has not been tested for home use.