A key to successfully making many hundred-year-old dishes (as well as many modern ones) is the ability to make a good white sauce. An article in a century-old magazine called it the mother sauce. Here’s some excerpts from that article:
The Mother Sauce
The mother sauce is merely a very-well-made white sauce. But tremendous importance is attached to the words well-made. When it is done, it should be creamy, ivory-tinted, smooth, a velvety liquid that clings, but does not stifle, blending its delicate flavor with and invariably enhancing that of the croquettes or vegetables with which it is served. But though the sauce be light and ethereal when rightly made, the making of it must be undertaken with concentration and seriousness.
Such a sauce is not often encountered – more’s the pity – but it is quite as simple to prepare as the less pleasing variety, and because of its many uses its secret should be mastered by every housekeeper. Thin, it provides the most delicious of dressings for vegetables, omelets, fish, and other dishes, or it forms the base of the most delicate of our cream soups and souffles. Thick, it is the foundation for the best of our croquettes, souffles, and dishes au gratin. And, with it as a background, any number of variations may be produced by the addition of flavors, herbs, or other condiments.
A perfect white sauce is made in the following manner. Mix together to a smooth paste two tablespoonfuls of butter and two of flour. Cook to a smooth, bubbling, semi-liquid consistency over a hot fire. Do not allow the mixture to brown, but see that the flour is well cooked. Now add slowly and carefully a cupful and a half of cold milk. Stir constantly until the boiling point is reached. Then season with a half-teaspoonful of salt and a dash of white pepper. If you have stirred the sauce conscientiously, it will be as smooth and delicate as you can possible desire. No straining will be necessary; but it will do no harm to pass the sauce through a fine sieve.
An unusually rich cream sauce is sometimes required. In that case make the sauce half milk and half cream, and it will be extraordinarily good.
Good Housekeeping (April, 1917)