For the roux, in a saucepan melt butter and add flour – cook for 2 minutes stirring every so often. In order to avoid lumps, use cool roux and boiling liquid or hot roux and cool liquid. Lukewarm liquid works just fine on hot roux. Pour liquid all at once in the saucepan, bring to boil and cook for 2 minutes whisking constantly. Remove from the heat, add cream if desired and an egg yolk to color the sauce. Season sauce with salt, pepper and nutmeg to taste.

A mornay sauce is an enriched béchamel with cheese.