Step #1 Put the butter, mustard, lemon juice, & stock in a small saucepan.
Step #2 Heat on medium.
Step #3 Before the sauce comes to a simmer, sprinkle the Wondra flour over it & whisk it in quickly.
Step #4 Simmer on very low for about 2 mins.
Step #5 Spice up with salt & pepper to taste.
Step #6 Keep hot until ready to serve, mixing in parsley just before serving, or keep put in the fridged until you need it; you can reheat it in the microwave if you like.
Step #7 If you want the sauce to be a bit thicker, allow it to simmer & reduce until it is right.