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It’s simple and quick to make, and you can prepare it ahead.
Add the remaining clarified butter to the pan.
Cook the onions until translucent, then add the mushrooms and fry until golden brown and the liquid has evaporated.
Season with thyme, salt and pepper.
Deglaze the pan with the veal stock and let it simmer briefly.
Stir in the mustard.
Mix the Schmand (or sour cream) with the cornstarch until smooth, using a splash of cold water or stock to prevent lumps, then stir it into the sauce.
Bring to a boil, stirring, until the sauce turns thick and creamy.
Finish with a pinch of sugar.
Return the cornichons and the beef, along with any juices, to the pan.
Warm everything through gently, but don’t boil it, or the tenderloin will turn tough.
Cook the spaetzle according to the package instructions and toss them in butter just before serving.