Put the mussels in a large saucepan with a splash of water. Cook over a high heat and remove to a plate as they open. Discard any that don't open.
When cool, remove the top shells from each mussel and place them on a baking tray.
Release the mussels and check for crabs. Place a mussel in each clean top shell. This makes for easy eating. Strain the cooking juices and spoon a little over each mussel. Cover and refrigerate if not cooking immediately.
Crumbs: combine the breadcrumbs, Parmesan, paprika, parsley and chilli flakes in a bowl. In another bowl, combine the butter, zest and garlic then pour over crumbs and stir well so they are all damp with the butter.
Preheat the grill to its highest setting.
Place a spoonful of crumbs over each mussel then drizzle with a little olive oil.
Place under the grill for 1-2 minutes until the crumbs are golden. Serve immediately.