Combine half & half, Dijon mustard, flour and salt in 1-quart saucepan. Cook over medium heat, stirring constantly, 3-5 minutes or until mixture begins to thickened. (Do not boil.)
Stir in green onions; remove from heat. Add butter, stirring constantly with whisk, until smooth.
Serve sauce immediately over chicken breasts or fish.
Add additional 1 tablespoon flour, if thicker sauce is desired.