Contemporary Béchamel recipes essentially consist of milk that is thickened with roux (a combination of butter and flour).
2 tablespoons unsalted butter
1/4 cup finely chopped onion
1 tablespoon all-purpose flour
1 1/2 cups whole milk
1/4 teaspoon salt
Dash of freshly ground white pepper
Freshly grated nutmeg (optional)
How to Make It
Place butter in a small saucepan over medium-low heat; cook until butter melts, stirring occasionally. Add onion to pan; cook 10 minutes or until tender (do not brown), stirring occasionally. Sprinkle flour over onion; cook 2 minutes, stirring occasionally.
Gradually add milk to flour mixture, stirring with a whisk until smooth; bring to a simmer. Simmer 10 minutes or until thickened, stirring frequently. Strain mixture through a fine sieve over a bowl; discard solids.
Stir in salt, pepper, and nutmeg, if desired. Serve immediately.
Mornay Sauce: Prepare Béchamel Sauce. Wipe pan clean with paper towels. Return Béchamel Sauce to pan, and place over medium-low heat. Add 1/2 cup (2 ounces) shredded Gruyère cheese, stirring until smooth. Serve immediately. Yield: 1 1/2 cups (serving size: about 2 1/2 tablespoons).