Chateaubriand refers to a recipe rather than a specific cut of beef. The recipe is said to be the creation of chef, Montmireil, for François-René de Chateaubriand a dignitary who served as an ambassador to Napoleon. Yet another theory is that the dish was created by restaurant Champeaux after a book was published by Vicomte de Chateaubriand L'Itineraire de Paris a Jerusalam. (1811)
Today the dish is typically made from a thick cut of fillet, grilled, then served with a sauce bearnaise. The older version referenced above was cut from a thick sirloin and served with similar sauce made from white wine, shallots and a touch of demi-glace.