Bechamel Sauce, also known as white sauce, is a basic sauce which is used as the base for other sauces, such as mornay sauce and cheese sauce. The basic ingredients are flour, boiled milk, and butter.
The following recipe is taken from The Cook's Decameron: A Study In Taste, Containing Over Two Hundred Recipes For Italian Dishes. This is part of a project that puts out-of-copyright texts into the public domain. This recipe reflects the cooking at the turn of the last century. Update as necessary.
Prepare a mirepoix by mixing two ounces of butter, trimmings of
lean veal and ham, a carrot, a shallot, a little celery, all cut
into dice, a bay leaf, two cloves, four peppercorns, and a little
thyme. Put this on a moderate fire so as not to let it colour, and
when all the moisture is absorbed add a tablespoonful of potato
flour. Mix well, and gradually add equal quantities of cream and
fowl stock, and stir till it boils. Then let it simmer gently.
Stir occasionally, and if it gets too thick, add more cream and
white stock. After two hours pass it twice slowly through a tamis
so as to get the sauce very smooth.
See also: Wikipedia Cookbook.