Add the butter and onion to a large saucepan over medium heat. Season the onions with salt, to taste, and cook until they become translucent and very aromatic. Add the flour and cook until the flour and butter resemble wet sand, about 3 to 4 minutes. Gradually, whisk in the milk until incorporated. Add the bay leaf and nutmeg and season with salt, to taste. Slowly bring the milk/flour mixture to a boil, stirring frequently to prevent scorching on the bottom. Be sure that the milk comes to a boil or the flour will not cook out and you will be left with a very objectionable starchy feeling on your tongue.
Once the sauce has come to a boil and has thickened, remove it from the heat. Discard the bay leaf. Stir in the grated Parmigiano, taste and adjust seasoning, if needed.
Use right away or refrigerate, covered, until ready to assemble dish.