Beef Stroganoff Sauce

Melt the butter in the pan used to cook the beef (or a fresh pan over medium-high heat). Add the shallots and cook for 2 minutes until softened and translucent. Add the sliced mushrooms in a single layer if possible — do not crowd or they will steam instead of brown. Cook undisturbed for 3 minutes, then stir and continue cooking for another 3-4 minutes until the mushrooms are deeply golden-brown and have released and then reabsorbed their liquid. The pan should look almost dry. Season with salt.

Pour all the beef stock into the hot pan at once. It will sizzle violently and lift the browned fond from the pan bottom — use a wooden spoon to scrape every bit of the dark caramelized residue into the liquid. This fond is the flavor backbone of the sauce. Increase heat to high and reduce the stock by half, about 5-7 minutes at a vigorous simmer. The sauce will deepen in color and flavor significantly as it reduces.

Reduce heat to very low. Add the Dijon mustard and stir it in. Add the sour cream in two spoonfuls, stirring to incorporate each addition fully before adding the next. Do NOT let the sauce boil once the sour cream is in — boiling will cause the sour cream to curdle and break, leaving grainy white curds in the sauce. Stir constantly over very low heat for 2-3 minutes until the sauce is smooth, uniform, and pale brown. Add freshly ground black pepper and taste for salt. Stir in the parsley just before serving.