Add the mussels to the pan. Next, add the lemon juice and then the Chardonnay to deglaze the pan, and then quickly cover with a lid to steam the mussels. Note: Do not open for at least 1 1/2 to 2 minutes so the mussels can steam.
Then, lift the lid and add the parsley (checking to make sure that there is enough moisture in the bottom of the pan to continue to steam the mussels). If everything is going properly, the mussels will have almost fully opened and there will be plenty of liquid still in the bottom of the pan to finish steaming the mussels. In that case, quickly cover the pan for another minute and a half. If the wine has evaporated and the mussels have not opened, add a small additional amount of wine, about a half cup more, and then re-cover the pan to cook out the additional raw alcohol taste and allow the mussels to fully open.
Once 3 to 4 minutes total have passed since the beginning of the cooking process, all of the mussels should have opened and their natural juices should have leaked out into the bottom of the pan, combining with the fresh herbs and vegetables to create a simmering broth. Reduce the heat to low and stir in the butter until melted.
Plate the mussels in a bowl and pour the sauce on top. Serve with your favorite toasted bread.