Preheat oven to 375°. Rinse lamb under running water, taking special care with head. Pat dry. Trim any excess fat, leaving a very thin layer to protect meat while cooking. Rub lamb lightly with 2 tbsp. of the oil.
Using a paring knife, make small incisions all over lamb. Cut about half of the garlic into thick slices and slip into the incisions with about half of the rosemary. Season lamb all over with salt. Sprinkle a little garlic, rosemary, salt, and pepper on head, cover brain with aluminum foil, and set aside.
Place lamb, bone side down, in a large roasting pan, leaving plenty of room for potatoes and head, both to be added later. Slightly crush remaining garlic, scatter over lamb, and roast for 45 minutes.
Place head, cut side up, in roasting pan. Quarter potatoes, toss with remaining rosemary and oil, and scatter around lamb, stirring to coat with pan juices. Pour wine over lamb and continue roasting, basting with pan juices, until meat is firm (it is traditionally served well-done), about another 45 minutes.
Remove lamb from oven, transfer to platter, and allow to rest for 20 minutes. Return potatoes and head to oven and cook until potatoes are tender, about 15 minutes. To serve, cut lamb at joints. Discard foil and add head to platter. Moisten meat with pan juices, adjust seasoning, and serve with potatoes.