In a pot large enough to accommodate the mussels heat 2 tablespoons butter and gently cook onions, shallots, celery and half of the parsley for about 5 minutes or until tender. Add the wine and bring to a boil. Season with pepper, add the mussels and raise the heat. When the steam starts to escape, shake the pot, keeping the cover tightly on to rotate the mussels inside. Cook them for 5 to 7 minutes or until they open. Discard unopened mussels and set them in a deep bowl. Pour mussel cooking liquid through a fine strainer into a saucepan and reduce quickly and slightly. Whisk in remaining 2 tablespoons of butter and parsley and spoon over mussels. Serve with warm French bread to sop up the juices.