Put the butter in a mixing bowl. Finely chop the parsley leaves, thyme, garlic and seeded chilli. Mash into the butter with the lemon juice, crumbs and parmesan. Season with salt and black pepper.
Put the prepared mussels (thoroughly cleaned and scrubbed, and checked for broken or dead ones) into a deep pan over a moderate heat, pour in the white wine then cover tightly with a lid. Let them steam for a minute or two until their shells open.
Lift the mussels out of the pan, pulling off the top shells as you go. Lay the mussels in the bottom shells flat on a baking sheet or oven-proof tray. Spread a teaspoon or so of the butter over each mussel then cook under a preheated grill till bubbling.
Recipe by George Hughes Fishmongers at https://www.georgehughesfishmonger.com/fish-recipes-to-warm-you-up-this-winter/