Preheat oven to 300°. Mince garlic and rosemary, then combine with two tablespoons of olive oil in a bowl. Set aside. Put one tablespoon of olive oil in the bottom of a baking dish, 9x13 minimum.
Peel potatoes and carrots, and cut them into chunks. Put the leg of lamb in the baking dish, then arrange potatoes and carrots around it. Sprinkle salt and pepper over the lamb and vegetables, then pour oil/garlic/rosemary blend over the lamb.
Roast until lamb reaches preferred doneness, 1-2 hours. (See note!)
Once leg of lamb is cooked, set it aside on a platter and cover it with foil. If the carrots and potatoes are not completely cooked, cover the baking pan with foil and return it to the oven for another 25 minutes. Regardless, let the leg of lamb rest a minimum of 20 minutes.
Slice the lamb thin and serve with roast vegetables.