Rinse the mussels in a colander and check them according to the instructions on the package
Heat some butter in a pan with a lid, add the mussels for about 3 minutes or until the shells are slightly open
Let them cool and remove one of the shell halves with a knife
Grate the parmesan cheese and lemon zest. Mix this into the breadcrumbs and finish with some black pepper
Put a pinch of herb butter in the shell with the mussel, sprinkle with the breadcrumb mixture
Divide the shells between a baking dish, put them in a preheated oven under the grill until golden brown. Do not leave them too long, they will become dry