Scrub the mussels and remove the beards. Rinse in cold water, changing the water several times, then discard any that are open and do not close when tapped against the edge of the sink.
Drain the mussels and put them in a large saucepan with a little water. Bring to a boil, then cover the saucepan, shaking occasionally, until the mussels are open – this will take 2 to 3 minutes. Drain well, then discard any that remain closed. Heat the grill to high heat.
Mix the breadcrumbs and zest. Remove a side from each shell, then spread a little butter on each mussel. Place the mussels on a baking sheet and sprinkle with breadcrumbs. Grill for 3 to 4 minutes until crispy.