Butcher's tenderloin by John Spottiswood.

This is the part of the diaphragm that hangs between the last rib and the loin. It's often ground in hamburger (or butchers just take it home), but some people claim that its grainy texture and intense flavor make it a first-rate steak (if marinated first). It's better known in France than in the United States, so you'll probably have to ask your butcher to set one aside for you.