1. 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.
2. 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.
3. 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).
Calories 67; Fat 5.2g (sat 3.1g, mono 1.4g, poly 0.3g); Protein 2.9g; Carb 2.3g; Fiber 0g; Chol 16mg; Iron 0.1mg; Sodium 92mg; Calc 97mg