1. Melt butter in a medium-size heavy saucepan over medium heat; whisk in flour until smooth. Cook 1 minute, whisking constantly. Gradually whisk in 1 1/2 cups milk; cook over medium heat, whisking constantly, 3 to 4 minutes or until mixture is thickened and bubbly. Remove from heat, and whisk in buttermilk and remaining ingredients.
Note: Sauce can be made up to two days ahead. Prepare recipe as directed; cover and chill in an airtight container. Whisk in 2 Tbsp. milk, and microwave at HIGH 1 minute, stirring at 30-second intervals.