Smear the butter in the chest cavity of the lamb. Sprinkle 1 tablespoon each of the salt, pepper, and oregano into the cavity. Squeeze the lemon into the cavity, spreading the juice with your fingers. Place the used lemon rinds in the cavity. If you have a very large refrigerator or cooler, wrap the lamb in plastic wrap and marinate for at least 4 hours and as long as 12