Fennel Sauce

A classic French liaison sauce built on velouté, finished off the heat with egg yolk, butter and lemon. Ranhofer assumes you have his numbered base preparations already made; the whole thing takes five minutes once you do.

• The hinge: never boil after the yolk goes inBring the velouté and glaze to a boil first, then pull the pan off the heat before whisking in the yolk. Temper the yolk with a ladle of hot sauce, return it, and hold the sauce below about 180°F — past that it curdles and no amount of whisking brings it back.
• Velouté (No. 415)A white roux slackened with veal, chicken or fish stock and simmered until it coats a spoon. Use a fish or chicken velouté depending on what the sauce is going with; fennel sauce most often accompanies fish.
• Meat glaze (No. 401)Glace de viande — brown stock reduced to a syrupy, near-solid concentrate. A commercial demi-glace or glace base works; a spoonful here means roughly a tablespoon, and it is there for depth and gloss, not for salt, so taste before seasoning.
• "A spoonful" and "some fresh butter"Nineteenth-century recipe shorthand. Read the spoonful as a tablespoon and the butter as 1 to 2 tablespoons, cold and cut in bit by bit at the end. Fresh here most likely means unsalted, a distinction that mattered more when much butter was salted for keeping.
• Which fennelThe tablespoon of very finely chopped fennel most likely means the feathery green fronds of the herb, not the bulb. Chop it at the last moment and add it off the heat — cooked fennel frond quickly turns drab and loses much of its anise scent.
• Lemon lastA whole lemon's juice into a pint of sauce is assertive and also thins it. Add it after the yolk, in stages, tasting; acid added to a hot dairy-and-yolk sauce is another route to curdling if the pan is too hot.
• Yield and holdingAbout a pint, enough for six to eight portions of poached or grilled fish. Hold it in a bain-marie no hotter than 150°F for up to half an hour; it will not reheat cleanly on direct heat.