Skills: White Sauce
Up until the early nineties you could go to almost any restaurant or any hotel and expect to see either a white-based sauce or a tomato-based sauce prominently featured upon the menu. However in recent years the white sauce has declined somewhat in popularity as huge numbers of international travelers and holiday makers have brought back a firm demand for world cuisine..
Regardless of this learning how to make a decent white sauce is still an essential skill for a chef. Simple additions to a white sauce will produce anchovy, bechamel, mornay, green, onion and cardinale sauce. From this simple, modest sauce a whole realm of new possibilities arise and if you do still hold true to classic cuisine this sauce is definitely for you.

There are two ways to make a white sauce. The first, which is traditional, is the correct way to make the sauce and is realistic for quantities up to one pint. Any larger quantities than this simply requires more hassle than it is worth, at this point I would then recommend using the cowboy method. See below!

Makes 1/2 pint of sauce

Ingredients
25g / 1oz flour
25g / 1oz butter
290ml / .5 pint milk
salt and pepper

Method:
In a saucepan melt the butter
Add the flour, stir for 1minute to mix the two ingredients together and allow time for the flour to cook
Slowly add the milk bit-by-bit
As you add the milk beat/ stir well to prevent lumps forming
As the milk heats the flour shall thicken the sauce
Continue to add all the milk, beating as you go
Allow the sauce to boil, then turn down to a simmer,
The sauce is thick enough when it will coat the back of a spoon, yet still leave a track when you pass your finger through it. (see images)
Season with salt and pepper

Cowboy Method:
For sauces with a volume greater than a pint:
In a saucepan bring all the milk to a simmer
In another saucepan melt the butter,
Add the flour to the butter, stir for 1minute to mix the two ingredients together and allow time for the flour to cook
Pour the simmering milk into the saucepan that contains the flour and butter
Blend
Bring to the boil, reduce to simmer,
Season

Notes:
The cooked flour and butter which has been added in equal quantities is known as a 'roux'.
You must beat vigorously to prevent lumps forming. If a lump does appear, remove the saucepan from the heat and beat until it disappears. If you continue to stir over the heat the lump will congeal and be exceptionally hard to remove!
You must bring the sauce to the boil. This will give the sauce a nice 'sheen'.
If there are any lumps in the sauce when you finish pass the sauce through a sieve to remove
If you use the cowboy method it is always wise to make more roux than necessary. Reserve some of this roux and you can then add to the milk and blend if the sauce is too thin

Adjust:
Too bland: add more salt and pepper
Too thick: add more milk
You can taste the flour: cook for longer

Science behind the dish: How flour thickens the sauce
Flour is largely constituted of starch molecules, as these are heated in the milk not only will the molecule swell but it will begin to unravel. It unravels in the form of elongated strands, these strands tangle with other similar, unraveling starch molecules to form a web or a lattice.
This web of molecules not only slows and traps the water molecules but also reduces the space that the water molecules can freely travel to and from. This is what causes the thickening of the sauce.