Preheat the grill to a moderate heat.
Discard mussels that are broken or do not close when tapped.
Steam the mussels in a large covered pan with boiling water for two minutes until they open.
Drain the mussels in a colander and allow them to cool.
Soften the shallot in 23g of butter inside a small pan.
Add the vermouth and cook until the liquid has evaporated, then transfer to a bowl.
Mix the shallots with remaining butter, garlic, walnuts, lemon, parmesan, breadcrumbs, and herbs.
Discard the top shells of the mussels.
Fill each mussel with 5ml of the stuffing and place them on a baking tray.
Grill for 3 minutes until the stuffing is golden-brown and bubbling.
Serve the mussels on a bed of rock salt with extra lemon.