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Onion Sauce
Cut two white medium onions in halves, then finely slice them, place in a saucepan with an ounce butter, and fry very slowly twelve minutes without browning, frequently stirring meanwhile. Add an ounce flour stir well while heating for one minute, moisten with a pint hot milk and season with half teaspoon salt, saltspoon cayenne and half saltspoon grated nutmeg. Sharply mix with whisk for one minute, then let boil for thirty minutes, frequently mixing meanwhile, remove and use as required.
A béchamel built on slowly sweated onions, thickened at roughly an ounce of flour to a pint of milk. The whole dish turns on the first twelve minutes.
- The hinge: no colour on the onionsKeep the heat low enough that the onions go translucent and soft without taking on any browning. Any colour at this stage tints the finished sauce grey-beige and brings a caramel sweetness that fights the nutmeg.
- A saltspoonA saltspoon is a quarter teaspoon. So the recipe asks for 1/4 teaspoon cayenne and 1/8 teaspoon nutmeg. A full quarter-teaspoon of cayenne in two cups of white sauce is assertive by modern taste; start with an eighth if you want the onion forward.
- Weights, in cupsAn ounce of butter is 2 tablespoons; an ounce of flour is a scant 1/4 cup. Read the pint as a US pint of 2 cups; if the book is British, an imperial pint is 20 fl oz and the sauce will be looser.
- Warm the milkThe instruction to moisten with hot milk is not decorative. Hot milk combines into the hot roux readily and is the standard practice for sauces of this type.
- Thirty minutes of boilingLong simmering was standard practice to cook out the raw-flour taste, and it also collapses the onion into the sauce. Use the lowest possible simmer, a heavy pan, and stir the bottom often, or you will scorch the milk solids. Skim any skin that forms.
- Texture choiceAs written this stays a chunky, rustic sauce. Blending it, or pushing it through a sieve, gives a smoother onion purée in the finer sieved style.
- Yield and useSomewhat under 2 cups once the thirty-minute boil has reduced it, enough for four to six servings. The recipe says only 'use as required'; onion sauce of this kind was commonly poured over mutton, tripe, boiled poultry or hard-boiled eggs.
Onion sauce in this form is a béchamel with onion cooked into it, and it sits at the plain end of a family that runs up through the French sauce soubise, a sieved onion and cream sauce generally credited to the kitchens of the Princes de Soubise in the eighteenth century. British cookery kept the coarser version and attached it firmly to mutton, an association that survived into the twentieth century: a sauce cook needed a small vocabulary of white sauces and this was one of the workhorses. The nutmeg is the old signature of a milk-based sauce, used in northern European kitchens since spice became affordable enough to be routine, and the cayenne is a long-standing English seasoning for pale sauces.