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Soubise Sauce
Chop very fine half a medium, sound white onion; place in a saucepan with half ounce butter and gently brown on the range for five minutes. Add two teaspoons flour, mix well; add one good gill boiling milk and mix well again. Season with one-quarter teaspoon salt and one saltspoon cayenne pepper. Boil slowly for ten minutes. Add one raw egg yolk and mix well without boiling. Strain the sauce through a cheesecloth over the dish.
A small béchamel built on onion and finished with a yolk. It makes a scant cup, enough to nap two or three portions, and it wants to be made just before serving.
- The hinge: no boiling after the yolkOnce the raw yolk goes in, the sauce must stay below a simmer or it will scramble into threads. Pull the pan off the heat, stir the yolk in briskly, and return it only to the barest warmth if it needs it.
- A gillHalf a cup, 4 fl oz. "One good gill" means a generous half cup rather than a level one.
- SaltspoonA quarter teaspoon. That is a great deal of cayenne for half a cup of sauce; a pinch to an eighth teaspoon suits most modern palates, and the sauce should read as onion first.
- "Gently brown"Classical soubise is usually kept pale so the sauce stays ivory. Five minutes in butter over medium-low heat should soften the onion and take the raw edge off it; if it colours much beyond gold, the finished sauce will be beige and slightly sweet-bitter.
- Half an ounce of butterOne tablespoon. With two teaspoons of flour that is a loose roux, which is why the sauce stays pourable rather than setting.
- Boiling milkAdding the milk hot, all at once, off the heat, and whisking hard is the reliable route to a lump-free sauce. Ten minutes at a slow boil cooks out the raw flour taste.
- StrainingCheesecloth over a sieve, or simply a fine sieve, pressing the onion through with the back of a ladle so its body stays in the sauce. Alternatively blend it smooth and skip the cloth.
- ServingAbout half a cup once it has boiled down and been strained. Traditional with mutton or lamb, poached or roast poultry, eggs, and boiled vegetables. It thickens fast as it cools and does not reheat well once the yolk is in.
Soubise is the standard French name for an onion sauce or purée, generally traced to Charles de Rohan, Prince de Soubise, an eighteenth-century marshal of France whose kitchen was famous enough to attach his name to the dish — a common pattern in French cookery, and one where the attribution is traditional rather than documented. In the classical repertoire it appears in two forms: a purée of onions cooked with rice, and the version here, a béchamel flavoured with onion and passed through a sieve. The yolk liaison used here is a common nineteenth-century way of enriching a white sauce.