Combine peeled garlic, rosemary, salt, and pepper in a food processor. Process until finely minced while pouring in the melted butter. Coat top and sides of lamb with the mixture. Allow to sit for 1 hour at room temperature. Preheat the oven to 450 degrees F (230 degrees C). Toss potatoes and unpeeled garlic together with olive oil. Place lamb in the center of a roasting pan; surround with potatoes and garlic. Roast in the preheated oven for 75 to 90 minutes. An instant-read thermometer inserted into the center should read at least 130 degrees F (54 degrees C). Let lamb rest for 20 minutes before slicing.