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Lemon Sauce for Fish
Squeeze and strain the juice of a large lemon into a lined saucepan, put in with it one-fourth pound of butter and pepper, and salt to taste. Beat it over the fire until thick and hot, but do not allow it to boil. When done mix with the sauce the beaten yolks of two eggs. It is then ready to be served.
A short lemon-butter emulsion of the hollandaise family, built in reverse order: the acid and fat are heated together first, the yolks go in at the end. It is quick, and it will break if it gets too hot.
- The hinge: never let it boilOnce the yolks go in the sauce holds together only within a narrow band of heat. Keep it below a simmer throughout, and pull the pan off the burner before stirring the yolks in — residual heat is enough to thicken them.
- A lined saucepanIn this period a 'lined' pan usually meant one lined with tin (commonly over copper) or with enamel, specified because bare copper reacts with acids such as lemon juice and taints the sauce. Any stainless steel or enamelled pan does the same job.
- QuantitiesOne-fourth pound of butter is 4 oz, one stick, 113 g. A large lemon yields roughly 3 tablespoons of juice, so the sauce is heavily weighted toward butter; taste and add more juice if you want it sharper.
- Work the butter in graduallyThe recipe puts the butter in with the juice at the start; cutting it into pieces and adding them a few at a time, beating constantly, tends to give a more stable emulsion.
- The eggsThe yolks are barely cooked by the warmth of the sauce. If you are serving anyone who should avoid undercooked egg, use pasteurized yolks, or return the finished sauce to very low heat and whisk until it reaches 160F.
- Holding itIt will not sit. Keep it in a bowl over warm — not hot — water for at most twenty minutes, or make it while the fish rests.
- Yield and servingAbout three-quarters of a cup, enough for four portions of poached or broiled fish. Spoon it over at the table rather than saucing the platter.
Oscar Tschirky's book gathers the working sauces of a large hotel kitchen, and this is one of the simplest: the same butter-acid-yolk logic as hollandaise, without the vinegar reduction or the double boiler, written tersely for cooks who already knew how such sauces behaved. Lemon with fish was by then long settled in French practice, and the group of warm emulsified butter sauces was being codified in these years — Escoffier's Guide Culinaire followed in 1903. The instruction to use a lined saucepan is a characteristic period detail: professional kitchens of the period made heavy use of copper, and acidic preparations were often given that warning.