To prepare the baked mussels recipe start heating the oven to 200°C /180°C fan, gas mark 6.
Scrub the mussels well with a small metal brush and wash under running water. Put into a large pan and cover with water, put a lid on the pan and bring to a boil over high heat.
Cook until the mussels open (throw away any that do not open). Drain (keep 1 cup of the cooking water and drain to remove any impurities), then remove the empty halves of the shells and place the halves containing the mussels on a baking tray.
Wash the parsley, shake dry and chop finely. Heat 45 ml of extra-virgin olive oil in a frying pan and fry the breadcrumbs, stirring, until golden brown. Mix together the parsley, chilli, breadcrumbs, Parmesan cheese, garlic, 30 ml of extra-virgin olive oil and the mussel water until you have a moist mixture that holds together. Season with salt and pepper.
Place a little of the mixture on top of each baked mussel and press down slightly. Put the baking tray into the preheated oven for about 2 minutes, until the gratin topping is nicely browned. Serve the baked mussels hot.