Make sure all the mussels are alive before you cook them – if any are open, tap them on the work surface, if they don’t close, throw them away.
Heat a large, wide pan, tip in the mussels with a splash of water, cover and cook until opened. Throw away any that won’t open. Remove the top shell from each mussel and discard. Mix the butter, breadcrumbs, garlic and parsley. Put a layer of half mussel shells on a baking tray. Sprinkle breadcrumbs over each one then flash under a very hot grill until just golden. Serve with lemon and crusty bread.