Start by melting your butter in a small/medium sauce pan over medium heat.
Once melted, add flour and whisk over medium heat for about one minute, making sure there are no lumps of flour.
Turn heat down to low and stream in milk while whisking continuously.
Turn heat back up to medium and cook for about 5 minutes while whisking continuously until the sauce begins to thicken. Once it coats the back of a spoon it is finished.
Remove from heat and stir in salt, pepper, wholegrain mustard, parsley and lemon juice.
At this point taste it and add any additional salt or pepper as desired.
Serve over seared salmon and veggies. Enjoy!