I use this sauce in loads of recipes, especially with pasta. To save me having to write it out every time I use it, here is how you make a basic white sauce:
Take: about 2 tbsp oil, about the same amount of flour, some milk (up to a pint).
1. Warm up the oil in a good-sized saucepan, then turn the heat right down. You may even want to turn the heat right off until you have everything under control.
2. Add the flour and mix thoroughly until it is cooked (i.e. it looks like paste), which should take about half a minute at the most.
3. Add the milk, little by little, stirring all the while, as vigorously as need be to avoid lumps. Each time you add the milk, mix it in thoroughly before adding any more.
NOTE: At first, each time you add milk, the paste will thicken suddenly as the flour and milk react together – so, to begin with, make sure you use the minimum amount of heat (the heat of the pan itself will usually be enough at the start); add only small amounts of milk at a time; and stir, stir, stir. Otherwise you will get lumps. If you get lumps anyway, a whisk will often sort it out but gets all stickety so don’t resort to this unless you have to!
4. Stop adding milk once you get to the desired consistency (depends on the recipe, but usually a good thick sauce is best).
Depending on the recipe, you may want to add some seasoning or other flavours. For example, to make a basic cheese sauce, add cheese. To make a basic mustard sauce, add mustard. Easy game.
INTERESTING CULINARY FACT
More or less, the proper name for this sauce is béchamel or, if you add cheese, mornay (there are actually lots of sauces with fancy sounding names that are pretty much just white sauce with something in it). I say “more or less” because correctly speaking béchamel and its variations are not quite as simple as my basic sauce and involve veal stock. Anyhow, if the recipe calls for béchamel, you can safely use this sauce and only the most foodiest of people will call you out. In which case, don’t invite them round for dinner any more.