In a pan melt the butter over moderate heat. Stir in the flour and mix together thoroughly. Pour in the milk and stirring constantly with a whisk, cook over high heat until sauce thickens and comes to a boil. Reduce heat and simmer for 3 minutes, then beat in the cream, mustards, vinegar, parsley, salt, and a few grindings of pepper. Serve at once.
This makes a rather mild sauce. To increase the flavor add more of the mustards.