That term always causes problems, at least to me.
Some authors state it is a cut of beef, nothing more.
Larousse Gastronomique defines Chateaubriand as a slice of very tender fillet steak about 1 1/4 inch thick. Although it states that when it was named it was probably cut from the sirloin served with a reduced sauce of white wine and shallots moistened with deni-glace and mixed with butter tarragon and lemon juice.
Turning to the North American Meat Processor's "The Meat Buyer's Guide", the term is said to be the center cut portion of the whole trimmed tenderloin, cooked and served in one piece (no mention of a recipe there).
Then I can cite other references that claim Chateubriand is a recipe and not a cut of meat at all.
But those recipes always seem to call for the dish to be made with meat from the tenderloin, have never seen a hamburger Chateaubriand at all, which it seems one should be able to make if it was just a recipe.
My version of Escoffier mentions Chateaubriand sauce (shallots, thyme, mushroom peelings, white wine, and veal gravy, prepared according to his directions and finished with butter). Heck, Escoffier I think would have finished corn flakes with butter.
But he states the Chateaubriand is from the tenderloin.
Escoffier kinda tries to use the tem both ways, in that it is a cut of meat, but there is also a sauce named Chateaubriand.
That leaves me in a bit of a quandry. Is it Chateaubriand if I take the appropriate cut, slater some ketchup on it and serve it?
Or can it only be called Chateaubriand if the appropriate cut of meat is served with the apropos Chateaubriand sauce?
I have no idea. Can only recommend you find a recipe you like and go for it.