Finely chop 8 cloves of the garlic and place in a pan large enough to hold lamb. Add 2 tbsp. of the oregano, 2 tbsp. of the rosemary, 6 tbsp. of the oil, 1 cup of the lemon juice, wine, and salt and pepper to taste. Mix thoroughly. Place lamb in marinade, turning to coat well on all sides, then cover and refrigerate overnight.
Crush 6 cloves of the garlic and remaining 1 tbsp. oregano and 1 tbsp. rosemary together with a mortar and pestle (or mince them finely together). Season to taste with salt and pepper.
Preheat oven to 375˚. Remove lamb from marinade, discard marinade, and pierce lamb in 8–10 places with a paring knife. Rub garlic–herb mixture over lamb, pressing it into incisions with your fingers, then coat surface of lamb with about 3 tbsp. of the oil.
Mix remaining 3 tbsp. oil and 1 cup lemon juice together in a bowl. Crush remaining 4 cloves of garlic, place in a large roasting pan, and add potatoes. Place lamb on top of potatoes. Roast, frequently basting lamb with lemon juice mixture, and occasionally turning potatoes and basting them with pan juices, for 1 hour and 45 minutes for well-done. Turn off oven. Remove lamb and allow it to rest for about 10 minutes, keeping potatoes warm in oven until you are ready to carve and serve lamb.