This is James Beard's Stroganoff, which takes thinly sliced tenderloin and cooks it in butter. Then you add a dash of white wine to deglaze and some spring onions, then a slurp of sour cream and a bit of Worcestershire sauce. Tenderloin and butter were born to be together, and the same is true for sour cream. Tenderloin is fairly lean, so even with a more marbled filet, it helps to add some extra fat. The key is to cook the sour cream at a really low temperature so as to not curdle the sauce—you only need it to heat through. Once the sauce comes together, toss the seared beef and its juices back in, then serve the whole mess over egg noodles.