The first thing you've got to know about béchamel is that it's what's known as a "mother sauce." (And no, a mother sauce is not a sauce that signs permission slips or helps you with your homework.) A mother sauce is one of the building blocks of classic French cuisine—a sauce used to make other sauces. There are five mother sauces, and of those five béchamel might be our favorite.
In its purest form, béchamel is comprised of butter and flour that have been cooked together (a mixture that's also known as a roux) and milk, with just a bit of seasoning. The result is a silky cream sauce that can be used either on its own or as the base for countless other sauces.
Béchamel is thick, clinging to food in the way that a good sauce should—and it’s that roux we have to thank for that. The first step in making a béchamel is to create a roux, a mixture of flour and fat that acts as a thickening agent. (If you've made gravy on Thanksgiving, you've probably made a roux before.) To make a roux for béchamel, you simply heat the fat—in this case butter—and whisk in roughly equal parts flour, cooking it just long enough to get some of the raw flavor out of the flour but not so long that it takes on any color. (In some other instances—gumbo being one of them—a roux will be cooked for an extended period of time, until it takes on a deep, dark color and nutty flavor, but that's not what we want here.)
Once the roux is properly cooked, the next step is to whisk in milk. (You may have noticed at this point that this sauce is pretty much just fat, flour, and more fat—that’s why it's so good, folks.) Milk is what turns the roux into a sauce. But you can’t just dump it all in at once. To create a smooth consistency (a necessity when it comes to béchamel), you need to gradually whisk in the milk. If you pour it all in at once, it will get clumpy and awkward. No one wants an awkward sauce. But we do want a flavorful sauce, so we need to continue to cook the béchamel, whisking it frequently, until it thickens to the degree that we want it to. Different recipes will call for different amounts of milk and different co*king times, which will produce different consistencies. For example, when making a croque monsieur, you want the sauce to be thick and almost spreadable, but for lasagna you want it to be thinner and almost pourable.
Once you've reached the degree of thickness you're after, it's time to season the béchamel with salt and nutmeg. Yeah: nutmeg! It’s not a flavor you should necessarily be able to pick out if you didn’t already know it was there, but nutmeg lends this otherwise kind of one-note sauce a considerable amount of warmth, spice, and complexity. It’s like putting a coat on over your favorite sweater. The whole situation gets cozier.
