1 Melt the butter, then add the finely sliced onions. Season with salt and freshly ground pepper, sweat for 15 minutes, then add the flour. Stir well and cook covered on very low heat, watching carefully that the bottom doesn't stick, for 45 minutes.
2 Add the vinegar and cook for another 10 minutes, then add the stock. Stir well and finish cooking covered on low heat for half an hour.
3 Taste and adjust seasoning. At the end, everything should have the appearance of cream.
4 This old recipe used to be made in the fireplace hearth, very slowly. This preparation replaced the mustard served with a roast or pork chops or grilled sausages, and even with a fillet of pan-fried cod, the result is guaranteed.
The key is to watch the onions cook during those first 45 minutes: the bottom must never stick to the pan, or the sauce will turn brown and taste burnt. Use a really gentle heat, cover it, and stir every 10 minutes. In Picardy, some old cooks add the vinegar in two stages: half after the onions have melted, the other half with the final stock. This spreads out the acidity and makes the sauce more complex. With a pork roast or pan-fried cod, serve hot in a separate sauceboat. A dry Alsace Riesling pairs perfectly: the acidity of the wine holds its own against the vinegar in the sauce, the melted onions echo the minerality of the grape.
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