There are a few things about Chateaubriand that most sources agree. It is a recipe, not a cut of meat. It was created for Francois René Vicomte de Chateaubriand (1768-1848) French author and statesman. It was created by his chef, Montmireil (possibly in 1822).
Sources differ on the other important details of this recipe. Most say it was originally cut from the thickest part of the beef tenderloin, but several state that it was originally cut from the sirloin. Some say it was one very thick cut of beef, seared on the outside and rare on the inside. It may or may not then have had the seared and charred ends cut off before serving. Others state that the thick steak (filet or sirloin) was cooked between two inferior steaks to enhance its flavor and juiciness. The inferior steaks were cooked until well charred, then discarded.
Most state that it was originally served with Bernaise sauce, but some say the sauce was made with reduced white wine, shallots, demi-glace, butter and lemon juice. Finally, all agree the steak was served with chateau potatoes (small olive shaped pieces of potato sautéed until browned).