Put the butter in a stewpan, set it on a moderate fire; when melted, sprinkle in it the flour, stirring the while; when of a brownish color, add the onions, salt, and pepper; stir, and leave on the fire till the whole is turning brown, then add the broth, boil about thirty minutes, and strain; mix well in a cup the vinegar, sugar, and mustard, which mix again with the sauce, and it is ready to be used.
Hand-Book of Practical Cookery (1884).