Peel four garlic cloves and gently simmer them in some of the milk you will be using for the garlic Parmesan cream sauce. When the garlic cloves are tender, strain them from the milk (reserving the milk) and use a garlic press to mash them.
In a heavy saucepan 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. Keep stirring until you have created a smooth sauce. Next add the milk, including the reserved garlic-flavored milk. Keep stirring while the sauce simmers. Stir the mashed garlic back into the sauce after adding all liquid to the roux.
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 wax paper or plastic wrap over the sauce while it cools to prevent a skin from forming.
Variation: For a roasted garlic Parmesan cream sauce, slice a head of garlic in half horizontally, sprinkle the cut halves with olive oil, wrap them in aluminum foil and bake them in a 375° F oven until the garlic cloves are soft and caramelized. Squeeze out six to eight of the softened garlic cloves and add them to the sauce.
Cooking Tip: You can roast garlic any time you are baking a casserole and have the oven on anyway.