Stuffed mussels is a favourite throughout Italy. In Tuscany alone there are at least four traditional dishes to my knowledge. This is how they cook 'em up in Livorno, with a tasty ground beef stuffing and stewed in a spicy tomato sauce.
This recipe can be made a day ahead of time and be reheated, or if you wish you can make the stuffing and the tomato sauce the day before. It can be served either as a main course (4-6 mussels per person) or as an starter (1-2 mussels per person). Do not clean and beard the mussels until you are ready to cook them. If you've made the tomato sauce the day before just add the mussel juice when you reheat the tomato sauce before you add the stuffed mussels.