Heat the broth until simmering. In a heavy sauce pan melt the butter until it is bubbling. Add the flour and stir well with a wooden spoon or spatula, creating a roux. Cook the roux for several minutes. Take care not to let it brown at all.
Slowly add the broth, stirring vigorously with a whisk to remove any lumps. Keep adding the liquid—first the broth, then the milk—until you have created a smooth sauce.
Add the nutmeg, ½ teaspoon of salt and pepper (or to your taste), and cook the sauce on low for 10 minutes, stirring frequently to prevent burning.
While the sauce is hot, add the Parmigiano Reggiano and stir until it is fully incorporated and melted.
Add the cream, if using, then correct the seasonings.
If you are making this sauce ahead of time, put a piece of parchment paper or plastic wrap over the sauce while it cools to
prevent a skin from forming.