Mussels to be served cold or stored overnight should be cooked in a saucepan over a moderate flame until all the mussels open.
A wide-bottomed pan,is preferable because it distributes the heat more evenly.
Shake pan continuously until mussels have released enough juice to prevent burning the pan.
As soon as all of them have opened, remove them; longer cooking will toughen them.
A few minutes should suffice.
If one or two stubbornly refuse to open, discard them as suspect.Garnish with chopped parsley,if desired.