2) Peel six of the cloves of garlic and place them in the bowl of a food processor fitted with the steel blade. Add the rosemary, one tablespoon of salt, one teaspoon of pepper, and butter. Process until the garlic and rosemary are finely chopped. Thoroughly coat the top and sides of the lamb with the rosemary mixture. Allow to sit at room temperature for 30 minutes to one hour.
Toss the potatoes and remaining unpeeled garlic in a bowl with the olive oil and sprinkle with salt. Place in the bottom of a large roasting pan. Place the lamb on top of the potatoes and roast for 1 1/4 to 1 1/2 hours, or until the internal temperature of the lamb is 57° (rare) or 62° (medium). Remove from the oven and put the lamb on a platter; cover tightly with aluminum foil. Allow the lamb to rest for about 20 minutes. Slice and serve with the potatoes.