Wash leg of lamb. Slit with sharp knife in various places on both sides of lamb. Slice garlic thinly and insert the slices in slits made in lamb. Season with salt and pepper and brush with melted butter. Squeeze lemon juice over lamb and place in roasting pan, fat side up. Roast at 450 F for 1/2 hour.
While lamb is browning, sprinkle potatoes with salt, rub with tomato paste, and let stand for a few minutes. Add potatoes to the baking pan with the water and lower oven temperature to 350 F. Turn and baste the potatoes occasionally during the roasting period. When lamb is done, remove to hot serving platter and keep warm. Increase oven temperature to 425 F and brown potatoes for 20 minutes longer.
