Wash and sort the mussels, pulling off any pieces of protruding beard. Combine the wine, shallots, bay leaf, and thyme in a 10-quart pot and bring to a simmer over medium heat. Simmer for about 5 minutes, then add the mussels, cover the pot, and turn the heat up to high. When steam starts to shoot out from under the lid, turn the heat back down to medium. Leave the pot on the heat for 5 minutes more. Leave the lid on the pot and holding down the lid with a kitchen towel- shake pot to redistribute the mussels. Put the pot back on the heat for 2 minutes more. Take off the lid and check to see if all the mussels have opened. If not, replace the lid and cook for 2 minutes more. Remove the lid, wait a minute for the steam to dissipate, scoop the mussels out of the pot with a large spoon or skimmer, and place them in hot bowls, discarding any unopened mussels. If the liquid in the bottom of the pan is sandy, carefully pour it into a clean saucepan, leaving any sand or grit behind. Add the chopped parsley to the hot broth, whisk in the butter if desired, and season with pepper. Heat the broth for a 1 or 2 minutes and ladle it over the hot mussels. Serve slices of crusty bread at the table.