Tenderloin Steak with Mushrooms

Ranhofer writes in shorthand: this entry is really a garnish and sauce built onto the tenderloin recipe next door (No. 1428), which is roasted or pan-seared whole fillet, sliced and dressed. Everything that needs decoding here is in the sauce and the mushrooms.

• Half-glaze"Half-glaze" is Ranhofer's term for demi-glace — brown espagnole sauce reduced with stock until it coats a spoon. A good commercial demi-glace concentrate, or veal stock reduced hard, stands in; the recipe gives no quantity, so judge it by the amount of meat and reduce until it naps.
• Mushroom essenceNo. 392 is the strained liquor from cooking mushrooms, reduced until intensely savoury. In a modern kitchen you get the same thing by saving the liquid the mushrooms release, reducing it to a few spoonfuls, and stirring it into the sauce at the end.
• Channeled mushroomsChanneling (canneler) means cutting shallow spiral grooves into the mushroom cap with the point of a knife for looks. Skip it without loss; use firm white or cremini caps, trimmed level so they sit on the meat.
• Cook the mushrooms first, then heat them in the sauceThe hinge of the dish. The recipe has the caps already cooked (No. 418) and only warmed through in the reduced sauce; a gentle cooking in butter with a little acid and water is the usual way to keep such caps pale. Simmering raw mushrooms in demi-glace muddies the sauce and turns the caps grey and slack.
• DonenessAim for an internal temperature of 125–130F for rare to medium rare, then ten minutes' rest before slicing. The sauce is the thing that should be hot and heavily reduced.
• YieldA whole tenderloin dressed this way serves six to eight as a course. Figure two to three mushroom caps per person.
• ServingThis is a plated-and-poured dish, not a braise. Have the platter warm, the sliced meat overlapped down it, mushrooms on top, and the sauce poured over at the last moment — reduced demi-glace loses its gloss as it cools.