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Sour Sauce for Fish
Heat a quarter of a pint of good vinegar, stir into it half a tea-spoonful of made mustard, a little pepper, and a slice of fresh butter. Serve when the butter is dissolved.
Four ingredients and no timings, so almost everything here is a question of translation. The sauce is essentially a warm vinegar butter, and the one thing that will spoil it is heat.
- A quarter of a pintCassell's works in imperial measure, so this is 5 fl oz — a scant two-thirds of a US cup. Halve it if you are cooking for two, as the sauce is sharp and a spoonful per plate goes a long way.
- Made mustard"Made" mustard means mustard already mixed with liquid, i.e. any prepared mustard from a jar, as opposed to dry mustard flour. English mustard is the period-correct choice and the most assertive; Dijon gives a rounder result.
- A slice of fresh butterAn imprecise Victorian measure, but a slice cut from a block is roughly 1 oz (25-30 g). "Fresh" is the important word: it means unsalted, since salted butter was the keeping kind. Vinegar already carries the seasoning here.
- Do not let it boilThe recipe says only "heat" and to serve as soon as the butter has dissolved, which suggests gentle treatment rather than a boil. Bring the vinegar to steaming, off the direct flame if need be, then stir in the butter until it melts. A hard boil will drive off the vinegar's aroma, and the butter is liable to sit as a greasy layer rather than staying mixed.
- Good vinegarChoose something you would be willing to taste on a spoon — cider or a decent white wine vinegar. Harsh distilled malt vinegar at this concentration is undiluted and unforgiving.
- What to serve it withThe recipe says only "for fish." A sour butter sauce of this kind suits oily fish best: herring, mackerel, sprats, grilled sardines. It is likely to overwhelm sole or a delicate white fillet.
- Yield and timingMakes about 5-6 fl oz, which should serve about four at a spoonful a plate. Best assembled in the last two minutes before serving, as it is unlikely to hold or reheat without separating.
Sharp sauces for oily fish are one of the oldest fixed pairings in European cooking — medieval and early modern kitchens reached for verjuice, vinegar or mustard whenever herring or mackerel came to table, on the reasoning that acid cuts richness and that strongly flavoured fish needs a strongly flavoured foil. Mustard with herring in particular is a pairing found across Northern Europe and still common in Scandinavian and Dutch cooking. The entry is of the short reference kind, where a cook was expected to find a formula rather than a lesson: no yield, no timing, no serving suggestion, just proportions to be scaled by judgement. The brevity suggests how routine warm vinegar-and-butter sauces were taken to be.