Steamed mussels served in sauce make an excellent first course to a meal, but they can be a bit messy. In this recipe, the mussels are served on the half shell for an elegant and easy-to-eat party hors d’oeuvre. Steam the mussels until they pop open, then remove one side of their shells. Pack on a topping of fresh breadcrumbs mixed with butter, garlic, and parsley and broil until crispy. Serve with a crisp champagne cocktail.