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Sauce Mousseline
Half gill sweet cream, two egg yolks, one ounce butter, two saltspoons salt, half saltspoon grated nutmeg and the strained juice of a sound lemon. Place cream, egg yolks, salt and nutmeg in a saucepan, place the pan in a "Bain-Marie," seeing that the pan is fully half its height in the hot water; briskly beat it up with a whisk for five minutes, then add the butter, little by little, whisking constantly until the butter is dissolved. Strain the preparation through a napkin into a saucebowl and serve as directed.
A cream-based cousin of hollandaise, thickened by whisking yolks over hot water rather than over direct heat. The measurements are small and the margin between thickened and scrambled is narrow.
- The measurementsA saltspoon is 1/4 teaspoon, so two saltspoons of salt is 1/2 teaspoon and half a saltspoon of nutmeg is a scant 1/8 teaspoon. An ounce of butter is 2 tablespoons. Half a gill is about 1/4 cup (2 fl oz) if the American gill is meant; on the imperial gill of 5 fl oz it would be nearer 2 1/2 fl oz, so treat the cream as a scant quarter to a generous quarter cup.
- Bain-marieA saucepan set in a wider pan of hot water, the way a double boiler works. Keep the water hot but not boiling; the sauce should stay under about 180F or the yolks will scramble and no amount of whisking will bring them back.
- The lemon is orphanedLemon juice appears in the ingredient list and is never mentioned again in the method. Whisk it in off the heat at the very end, a little at a time, tasting as you go. Adding acid early makes curdling more likely.
- Butter, little by littleThis is the hinge. Soften the butter and add it in small pieces, letting each one disappear before the next goes in. Dumping it in cold and whole drops the temperature and the emulsion breaks.
- Five minutes is a guide, not a ruleWhisk until the mixture thickens enough to coat the whisk and hold a brief ribbon. Depending on your pan and water temperature this may take rather more or less time than stated.
- Straining through a napkinA fine-mesh sieve does the same job. Do it while the sauce is warm, and use a warmed bowl, since the sauce sets and dulls quickly as it cools.
- EggsThe yolks are cooked, but gently and briefly. If you are serving anyone who should avoid undercooked egg, use pasteurized yolks.
- Yield and serviceAbout half a cup, enough for two to four servings. It is a sauce for poached fish, asparagus or other green vegetables, and it will not hold: make it immediately before it goes to the table and keep it barely warm.
Mousseline is generally taken to mean a sauce lightened to the texture of muslin or foam, and the name is usually attached to hollandaise enriched with whipped cream. This version reaches the same place by a different road: no separate hollandaise, just cream and yolks whisked together over hot water with butter beaten in at the end, closer in method to the older egg-and-cream liaisons used to thicken French sauces. The insistence on a bain-marie, and on the pan sitting half its depth in the water, points to a writer familiar with professional practice, though the text itself tells us nothing about who wrote it or for whom. Nutmeg in a savory cream sauce, which reads as unusual now, was common in the period and survives in bechamel.