In a 1 quart sauce pan over medium heat, combine heavy cream, Dijon mustard, flour and salt.
Stir constantly, for 3-5 minutes until mixture begins to thicken. Do not allow to boil.
Stir in green onions, then remove from heat.
Add butter 1 tablespoon at a time so it slowly emulsifies, whisking vigorously, until smooth.
Serve sauce immediately over chicken breasts or fish.
At Room Temperature - Keep at room temperature while using, but transfer to the refrigerator after a couple of hours.
Refrigerate - Store in the refrigerator in a covered container for up to a week.
Freeze - Let cool then store in an airtight container. Freeze for up to a couple of months. Note that because this sauce contains cream and has been emulsified, the consistency and texture may not be smooth and creamy like the original sauce.