Prepare mussels and inspect them carefully.
Place in a pan with onion, sprig parsley, water (slightly less water if adding white wine) and seasoning.
Heat gently until mussels open.
Take off shells and be careful to remove 'beard', i.e.
the rather stringy part, from mussels.
Meanwhile make a white sauce with the butter, flour and milk.
Strain mussel liquid into this, add white wine, if used, and reheat.
Put in mussels, heat gently and serve garnished with chopped parsley.