Bechamel & white sauces
A roux is fat meeting flour at heat, and a bechamel is a roux meeting milk. The whole sauce is the ratio and the order: the flour cooks out its raw taste in the roux, the milk thickens the roux into a film, and the one ingredient that keeps the whole thing from tasting like a paste is the butter, added early and in real quantities.
Why the order is the order
You can't add milk to raw flour and expect it to thicken: the starch hasn't gelatinized, and the liquid will run off the starchy particles in a paste that tastes like wet paper. You have to cook the raw taste out of the flour first — that's the roux — and then the starch in the flour is ready to grab the milk and hold it as a film. The fat in the roux does two things at once: it coats the flour particles so they don't clump when the milk hits, and it carries the flavor of the butter all the way through to the finished sauce. A bechamel with no butter in the roux is a paste, not a sauce, and the one way to fix it after the fact is to add butter at the end, which is exactly what a restaurant does when the sauce needs finishing.
The ratio
A standard white sauce is 25 g butter, 25 g flour, and 250 ml milk — the "two and a fifth" of the three. That's a sauce blanche: thick enough to coat a spoon, thin enough to pour over a pasta or a baked dish. A roux is the butter-and-flour part of that, cooked for 1–3 minutes until raw is gone and the color is the same pale gold as the finished sauce. The flour is the thickener; the butter is the carrier; the milk is the body. Change any of those three and you have a different sauce.
The moves
- Melt the butter over medium-low, add the flour, and stir for 1–3 minutes. You're not browning this roux (that's a different sauce, and a different dish) — you're cooking out the raw. It should smell of toasty fat, not of paper. A pale, uncooked roux in the finished sauce is the single most common "why does this taste like flour" answer.
- Add the milk in three parts, beating the first in thoroughly before the next goes in. The first third of the milk hits the dry, hot roux and will clump if you beat too slowly or add too much at once. The second and third thirds go in more easily. The target is a smooth, slightly thick, pourable liquid at the end of 5 minutes of gentle cooking.
- Season at the end, not the start. Salt and white pepper into a bechamel go at the very end, after the sauce has thickened and cooled a little. The salt at the start will keep you guessing the finished flavor, and the pepper at the start will be the first thing you lose to the heat. Season to taste, not to the recipe.
- Keep it moving until it's off the heat. The moment you stop stirring and the sauce is at a rolling simmer, the bottom will start to brown before the top has thickened, and the whole pot will stick. A wooden spoon, or a spatule, and a light circular motion, is the difference between a smooth sauce and a lumpy, scorched one.
From white sauce to cheese sauce
A bechamel with cheese stirred in is a mornay, and it's the base of anything that wants a creamy, cheesy, baked-on-top texture. The cheese goes in off the heat — the same rule as the miso in sauces-101: a cheese that's been in a rolling boil will break, string, and separate, and the sauce will go from creamy to oily in a sentence. Take the pot off the heat, let it cool for 30–60 seconds, then add the cheese in a few turns of the spoon until it melts into the sauce. Lobster mac & cheese is that dish, and the brown-butter version of the roux is the upgrade that a restaurant does over the plain white-sauce version at home.
In practice
Lobster mac & cheese is the whole page: the brown butter is the fat in the roux, the mornay is the sauce, and the baked-on-top is the finishing move that a restaurant does at the pass. Brown butter is the fat article to read before you build the roux for either of these dishes.