Do not bring Alfredo sauce to a boil. You risk two things if you do.
First, you risk scorching the cream which will damage your sauce and make it unusable.
Second, it's impossible to test for the proper thickness in a boiling sauce. The bubbles and temperature make the sauce seem thinner than it will really be once cooled and served. A sauce just below simmer gives a more reliable test for texture.
A good cook develops patience.
Authentic Alfredo Sauce
4 Tbs butter
1 cup heavy cream
1 cup grated Parmesan (Parmigiano Reggiano)
dash of nutmeg (the secret to good Alfredo Sauce
Melt the butter over a low heat in a pan that will be large enough to hold the pasta at the end of the cooking. Do not let it boil, just melted and warm is fine.
Slowly add the cream and stir with a spoon until it is hot but not boiling and has reduced slightly.
Now add the Parmesan cheese a little at a time and stir until it is all melted. The sauce may take quite a while to thicken, keep stirring and don't let it boil. Add seasonings according to your taste. If the Parmesan is grated too course it will not melt in well. Grate the cheese as fine as possible for best results. The sauce can take a long time to reduce. Don't be tempted to add flour or cornstarch for thickening as it alters the flavor.
4 oz. of cream cheese may be added for additional creaminess.
"Food is our common ground, a universal experience." - James Beard