Not sure if your butcher is playing fast and loose with the name, but a true Chateaubriand is not too big and typically only is large enough for two nice fillet-type cuts. May I suggest you treat them as you would a fillet as I’m sure they will be tender. Serve with a mushroom sauce or a Bearnaise sauce.
My fancy potato balls were made using a melon baller and then browned in a frying pan in which I keep them rolling!
Ron