Montmireil, the chef of the Breton vicomte and writer François-René de Chateaubriand (1768-1848), named this steak after his boss, who was then serving as French ambassador to beef-loving England.
Montmieil kept a large, boneless cut of beef moist while roasting, by sandwiching it between two fatty steaks instead of wrapping it in barde. When the steaks were charred, he knew the beef wrapped inside was done—and threw away the steaks. One hopes the dogs at least got some of that action!
Chef Montmireil also came up with pommes de terre château—potatoes cut into the shape of olives, sautéed in butter.
Today a châteaubriand is a thick tenderloin steak (generally 3cm or so), seared to seal in the juices, then constantly basted in butter as it cooks at a low temperature until done inside (by preference, quite rare) and served with sauce béarnaise.
Images by Anne-Louis Girodet de Roussy-Trioson , Open Food Facts