Combine the whole milk, Dijon mustard, flour, salt, and onion powder in a pan over medium heat. Stir it constantly while it begins to thicken for about 5 minutes.
You don’t want the mixture to boil, so make sure to pull it off the heat if you see bubbles.
Add the butter, stirring constantly until it melts and the sauce is smooth.
Notes
Enjoy your sauce with your Easter ham or with any meat or poultry dish.