You will find versions of this invaluable little sauce all around Bordeaux. It goes with almost any meat, from beef steak to lamb to pork chops to duck breast, as well as with such vegetables as celery root. Versatile indeed! A fruity red wine, such as a Merlot or a light Bordeaux blend, is best.
Melt the butter in a saucepan over low heat. Add the shallots, season with salt and pepper, and press a piece of aluminum foil down on the shallots. Cover the pan and sweat the shallots over very low heat, stirring occasionally, until they are very soft but not brown, 10 to 12 minutes. Discard the foil, pour in the wine, and boil, stirring occasionally, until the shallots are nearly dry, 15 to 20 minutes.
Whisk in the flour and cook, stirring, for about 1 minute. Whisk in the broth and bring the sauce to a boil, stirring constantly until it thickens. Boil the sauce until it coats a spoon and is reduced by about half, 10 to 12 minutes longer.
Take the pan from the heat and whisk in the cold butter to enrich the sauce and mellow the flavor. Taste and adjust the seasoning, adding the Cognac if you like. This sauce keeps well in the refrigerator for up to 3 days. Reheat gently before serving.
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