I have two articles on the back burner at the moment, and both of them use roux. Roux is a basic that every cook should know about, but for various reasons it's rather shrouded in myth.
Roux is basically a mixture of flour and oil, which are brought together to become a thickening agent for liquids. It is used for anything from gravy, stews, soups and various sauces. The most commonly used oil is butter, clarified or not.
Now about the myths: during the 1970s and 1980, when Nouvelle Cuisine became the rage, roux fell into disfavor. Roux-based sauces were regarded as being symbolic of "old-style" "heavy" traditional cooking. Instead of using roux, chefs used other methods for thickening and emulsifying their sauces. So here are the top three myths about roux:
One of the best known, and much maligned, roux-based sauces is a bechamel - otherwise known as "white sauce". It's a flavored milk sauce thickened with roux. If you have never mastered bechamel, here is how to get it perfect every time.
You will want 1 tablespoon of unsalted butter to 2 tablespoons of plain white flour. If you want to be even more precise, you want 1 weight unit of butter to 1 weight unit of flour (say, 10 grams - 10 grams), but I find that the 1 Tbs - 2 Tbs ratio works fine and is much easier to measure.
1 Tbs butter + 2 Tbs flour will thicken 1 cup of liquid, to produce a sauce of the consistency that is perfect to use as a pouring sauce, and for making lasagna and gratin dishes. It's also the basis for Sauce Mornay, otherwise known as plain old cheese sauce. So, if you want 4 cups of sauce, you'll use 4 Tbs + 8 Tbs, and so on. If you want a stiffer sauce, say for making cream croquettes, you would use more roux.
Melt the butter over a medium-high heat in a heavy-bottom pan. Add the flour, and stir around vigorously. It wil first turn creamy looking, then start to look a little grainy - this means that the flour granules have absorbed the butter. For a bechamel, stop right here and take the pan off the heat before it starts to turn brown. This is the basic light or "blonde" roux you want for thickening liquids in an unobstrusive way. The longer you cook a roux, the darker it gets, and stronger tasting - a toasty, deep taste. Dark roux is used for Cajun/Creole dishes like gumbo, for example. You can also use dark roux to color up a pale gravy.
To make 4 cups of bechamel, put 4 1/2 cups of milk (whole if you want the richest sauce, but you can also use low-fat..skim really doesn't work well) in a heavy-bottom pan. The extra 1/2 cup is to account for evaporation. Throw in 1 whole peeled onion stuck with 1 clove, and 1 bayleaf. Bring this up to heat and simmer for a while (at least 15 minutes) so that the milk becomes steeped with the flavors of onion-bayleaf-clove.
When the milk is piping hot and suitably steeped with flavor, make the roux using 4 Tbs butter + 8 Tbs flour, as described above. Now add the milk to the roux, one ladle at a time (straining out the flavoring incredients), and mix vigourously until the roux and liquid are amalgamated. Do not add the next ladleful until the mixture is smooth. Continue adding the liquid until the sauce is the thickness you require. Season with salt and pepper (white if you must have a pure white sauce, but I always just use black) and a little grated nutmeg
What if your bechamel (or any other roux-based sauce or gravy) is lumpy despite all your precautions? There is one thing that will fix any lumpiness: an immersion or stick blender. It's not just for pureeing veggies in soups or whirring up your powdered protein drinks! A few seconds of blending with this tool will de-lump your sauce in no time. A basic stick blender such as this one  doesn't cost much, takes up very little counter or drawer space, and is endlessly useful. If you don't have a stick blender, you can try to get as many lumps out of as possible by vigorously mixing the sauce with a whisk, and then if you want a perfectly smooth sauce, simply strain it through a sieve.
Now, you have conquered roux and bechamel - what to make with it? Stay tuned.
Addendum 1: for a very traditional bechamel, the onion (or shallot) is sautéed in butter along with a small amount of chopped veal, but I find that for most modern dishes requiring bechamel this is not really necessary.
Addendum 2: In Japan, 'white sauce' is available in cans. I don't know why other countries don't have canned bechamel, since t's so useful.