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Scrub the mussels and pull off any beards. Rinse in several changes of cold water, then discard any that are open and do not close when tapped against the side of the sink.
Drain the mussels and put in a large pan with a splash of water. Bring to the boil, then cover the pan, shaking occasionally, until the mussels are open – this will take 2-3 mins. Drain well, then discard any that remain closed. Heat grill to high.
Mix the crumbs and zest. Remove one side of each shell, then spread a little butter onto each mussel. Set on a baking tray and sprinkle with crumbs. Grill for 3-4 mins until crunchy.