Melt the butter in a heavy-bottomed saucepan over medium heat.
Add the sliced onions, cover the saucepan, reduce heat to low and sweat the onions for about 5 minutes or until soft and transparent.
Add the Béchamel and mix well to combine. Simmer on low heat for about 10 minutes.
Either strain the sauce into a clean saucepan, pressing out the onions to extract maximum flavor, or blend them in the saucepan with the sauce so they are puréed.
Add the cream. mix well and simmer over minimum heat, stirring continuously, for about 6 minutes or until the sauce has reduced and thickened.
Season to taste with salt, pepper and nutmeg.
Strain the sauce into its serving dish or sauceboat, again pressing out any solids.