Melt the butter in a large saucepan over medium heat. Add the flour and cook, stirring with a wooden spoon, to make a light roux, about 2 minutes. Whisking constantly, slowly add the milk and continue to cook, stirring occasionally until thickened, 5 minutes. Add the salt, pepper, nutmeg, and Parmesan and cook, stirring, until thickened, about 2 minutes. Remove from the heat. Place a piece of plastic wrap directly on the surface of the bechamel until ready to assemble the lasagna.
Yield: about 2 quarts