To make the butter, place all the ingredients up to and including the butter into a food processor. Blend to a smooth consistency, season with lemon juice, salt & pepper. Mix in the pepper and set aside.
Preheat the grill to a high temperature. Rinse the mussels under cold water and gently slide a knife under the mussel meat to remove the mussels from their shells. Pat dry both shell and mussel with kitchen paper. Drizzle a little oil into the shells. Place the mussels back into their shells and dot the garlic butter over the mussels and sprinkle with a little parmesan. Cook under the grill for 2-3 minutes.
Alternatively cook in a preheated oven at 230ºC/445ºF/Gas 8 for 3-4 minutes. Remove from oven sprinkle with chopped parsley and serve immediately.