Make a roux based white sauce, finely chop about half cup of parsley, put it in a clean tea towel like a ball and rinse it under the cold water until the water runs clean.
Add it and a tablespoon (or to taste) of Seeded Mustard to your sauce, salt and pepper and a touch of nutmeg, cook out the sauce stirring well on low heat until the floury taste is gone.

Roux Sauce:
100gm butter : 100gm Flour : 1 litre milk.

HTH