The main ingredient in traditional béchamel sauce is milk. This classic French sauce, one of the five mother sauces, is made by combining milk with a white roux, consisting of equal parts flour and butter. The resulting creamy, smooth sauce serves as a base for various dishes, including lasagna and croque monsieur. Béchamel is versatile and can be seasoned or enhanced with other flavors, making it essential in traditional and contemporary cuisine.