Cook the mussels, covered, until they open. Remove them from their shells and cool. In the meantime, mix the butter, parsley, garlic, salt and pepper together with a fork. Grate the stale bread and add it to the mixture. Fill each half shell with a mussel and spread the butter mixture on top with a butter knife. Preheat the oven to 425F. Place the filled shells on a baking sheet and heat for 10 to 12 minutes. Serve very hot.
