For preparing the filling you will need butter, breadcrumbs, garlic, salt and a little bit of lemon zest for a flavour. Mix everything together and distribute the filling in the shells. Then return mussels back and put them in a preheated oven on 150 degrees and check it frequently. When the butter melts and starts boiling, you can already sprinkle Parmesan and put them back in the oven for 1 minute, or until Parmesan melts (This also depends on your preferences). Entire baking process takes 10-12 minutes.