Mushrooms and beef have long been an inspired combination, and here they're joined in a particularly sophisticated way. Dried porcini mushrooms grind to a powder in no time in a blender and add a subtle woodsiness to the beef, while the truffle butter enriches the sauce with its heady aroma. Black-truffle butter is the best deal going. It gives you tremendous flavor at an affordable price, a fraction of the cost of a fresh black truffle.
•Whole tenderloins can come in various stages of trimming, particularly those from big-box stores. Some still have the chain attached—a long, thin, loose muscle hanging onto one side of the tenderloin—which you'll want to remove and save for something like kebabs. (Either ask the butcher to do the honors or do it yourself.) Whether the tenderloin comes trimmed (a.k.a. peeled) or not, make sure there is no fat or silverskin left on the outside. If there is, remove it yourself with a sharp knife.
•Porcini mushrooms can be ground 1 week ahead and kept in an airtight container at cool room temperature.
•You can find black-truffle butter at specialty foods stores, or order it online from Amazon or D'Artagnan.
•Crimini mushroom mixture can be made 1 day ahead and refrigerated once cooled. Reheat before proceeding with recipe.
