Using kitchen string, tie the parsley and thyme sprigs with the bay leaf to make a bouquet garni. Melt the butter in a large enameled cast-iron casserole. Add the bouquet garni, shallots, leek, celery, carrot, garlic, 1/2 teaspoon each of salt and pepper and the cayenne. Cook over moderate heat, stirring occasionally, until the vegetables are softened, 6 to 8 minutes. Add the wine and boil until reduced by half, 4 to 5 minutes. Add the mussels, cover and cook, shaking the casserole occasionally, until the mussels are wide open, 4 to 6 minutes.