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Mustard Sauce
Blend together on a plate three ounces of butter with a dessert-spoonful of browned flour, half the quantity of the best mustard, and a little salt. Stir these ingredients, when smoothly mixed, into a quarter of a pint of boiling water, and simmer five minutes. Add enough vinegar to flavour, and serve as a sauce for fresh herrings.
A quick butter-and-flour sauce thickened in water rather than milk, built to cut the oiliness of fresh herring. The whole thing takes under ten minutes.
- Quarter of a pintAn imperial quarter-pint is 5 fl oz, about 150 ml or a scant 2/3 cup. Using a US 4 fl oz measure will give a noticeably stiffer sauce.
- Dessert-spoonfulRoughly two teaspoons, sitting between a teaspoon and a tablespoon. "Half the quantity" of mustard therefore means about one teaspoon.
- Browned flourFlour dry-toasted in a pan or a low oven until biscuit-coloured, once kept ready in a jar. It thickens less than raw flour and adds a nutty, faintly roasted note; plain flour works but the sauce will be paler and thicker.
- The best mustardAlmost certainly dry mustard powder rather than a jarred paste, since it is worked dry into the butter and flour on the plate. If you substitute prepared English mustard, use about double and add it off the heat, since its flavour is already developed and will dull with cooking.
- The hingeWork the butter, flour, mustard and salt to a completely smooth paste on the plate before it meets the water. Any lump of dry flour at this stage stays a lump in the finished sauce, and there is no milk or roux stage to rescue it.
- SimmeringFive minutes at a bare simmer is right; it cooks out the raw flour taste. Keep it gentle and stir, or the butter will separate out into an oily film.
- Vinegar lastAdd the vinegar off the heat and by the spoonful, tasting as you go. Malt vinegar suits herring; two to three teaspoons is usually enough for this quantity.
- YieldRoughly 3/4 cup of sauce; the recipe states no yield or number of servings.
Mustard with herring and mackerel is a long-standing pairing in English cookery, on the same logic as gooseberry with mackerel: a sharp, pungent foil for a dark, oily fish. The sauce follows the common pattern of butter and flour thinned with water rather than milk or stock. Browned flour of this kind was often prepared in quantity and kept ready for quick brown sauces. Prepared mustard in jars existed by this date, but dry mustard powder, sold by Keen's and by Colman's, was a standard cook's form, mixed fresh as needed.