Thoroughly clean the mussels. Then cover with water and steam in a large saucepan until the mussels have opened. Then drain and remove the upper shell (discard closed mussels!). Put them in a baking dish or baking sheet so close together that they don`t tip over.
Preheat the grill to the highest level.
In a bowl, mix the butter with the olive oil, parmesan, parsley, garlic and black pepper. Put a small pile of this cheese mixture on each mussel.
Grill in the oven for 3-4 minutes until the mussels sizzle and are golden yellow.
Serve with white bread and lemon wedges.
Note: If you take frozen Greenshell mussels, wash them off with cold water, let them thaw and bake them under the hot grill at 180 ° C for about 7 minutes.