Comment: A basic sauce for many preparations. Typically butter, flour and milk, the liquid may, depending on the case, be a broth or a fish stock. The consistency of the sauce depends on the ratio of the relative amount of liquid, adjusted according to the need of the recipe. Typically, for 1 l of liquid, there are 70 g of butter and 70 g of flour. The amount of butter and flour is normally the same. In some cases, you can slightly favor butter, never flour. The Béchamel sauce will be more liquid for lasagna (which needs liquid for pasta), or thicker (for example for a soufflé).
The seasoning, is normally salt, pepper and nutmeg. Other elements are considered depending on the purpose of the sauce: bay leaf, thyme, garlic, capers, cayenne pepper and of course various cheeses.
Exemple, for 4 to 6 servings, sauce Mornay for raviolis : 30 g flour, 30 g butter, 5 dl milk, 150+ g cheese. A little more milk than usual because of the cheese.
Use of broth or stock (fish): we speak of "velouté"
Addition of onions (caramelized, then mixed): Soubise sauce
Addition of egg yolk (opt) and cheese: Mornay sauce
According to recipe, butter and flour
According to recipe, milk or other liquid
Melt butter over low heat.
Add the flour all at once and mix with a whisk to obtain a roux.
Cook without browning for 2 to 3 minutes.
Off the heat add the milk (cold - room temperature) all at once.
Beat vigorously and return to the heat.
Season (S + P + nutmeg).
Heat slowly, stirring, until the mixture begins to thicken.
Cook (without boiling) another small minute.
Remove from the heat and add the cheese if necessary.