What is the key ingredient in a classic béchamel sauce?

The key ingredient in a classic béchamel sauce is milk. Béchamel is one of the French mother sauces, and its foundation relies on the combination of milk with a roux, which is made from equal parts flour and butter. While flour and butter are essential for thickening the sauce, it is the milk that ultimately defines its creamy texture and rich flavor. In a béchamel, the milk is gradually whisked into the roux, allowing it to thicken as it cooks. This process creates a smooth sauce that serves as the base for many other sauces and dishes in classical cooking.

Including cheese, although common in variations like cheese sauce, is not part of the traditional béchamel preparation, which focuses purely on the combination of flour, butter, and milk.