Preheat oven to 450. Put oven rack in lower third of oven so lamb will sit in middle of the oven.
Peel 6 of the garlic cloves and put them in the bowl of food processor fitted with steel blade. Add rosemary, 1 Tbsp. salt, 1 tsp. pepper and butter. Process til garlic and rosemary are finely minced.
Thoroughly coat the top and side of the lamb with the rosemary mixture. Allow to sit at room temperature 30 minutes to 1 hour.
Toss potatoes and remaining unpeeled garlic in a bowl with olive oil and sprinkle with salt. Put in bottom of large roasting pan. Put lamb on top of potatoes and roast for 1 to 1 1/4 hours or til internal temperature of lamb is 135 (rare) or 145 (medium).
Remove from oven an put lamb on platter, cover tightly with foil. Allow lamb to rest for about 20 minutes. Slice an serve with the potatoes.