Preheat the oven to 450 degrees F. Place the oven rack in the lower third of the oven so the lamb will sit in the middle of the oven.
Peel 6 of the cloves of garlic and place them in the bowl of a food processor fitted with the steel blade. Add the rosemary, 1 tablespoon salt, 1 teaspoon pepper, and butter. Process until the garlic and rosemary are finely minced. Thoroughly coat the top and sides of the lamb with the rosemary mixture. Allow to sit at room temperature for 30 minutes to 1 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. Roast for 20 minutes. Turn the heat down to 350 degrees F and roast for another 1 to 1 1/4 hours, until a meat thermometer registers 130 to 135 degrees F for medium-rare. Place the lamb on a cutting board, cover with aluminum foil, and allow to rest for 15 minutes. Slice and serve with the potatoes.
This recipe has been updated and may differ from what was originally published or broadcast.
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