Melt the butter in a saucepan over moderate heat. Add the flour and mix thoroughly.
Add the milk and cook over high heat, stirring constantly with a whisk, until the sauce thickens and comes to a boil. Reduce the heat and simmer for 3 minutes.
Beat in the cream, vinegar, dry mustard, Dijon mustard, dried parsley, salt, and a few grinds of pepper. Serve immediately.
Recipe Tips
If you want to use fresh parsley, please use 1 tablespoon of chopped fresh parsley.