Put the mussels into a large pot with a splash of white wine over moderately high heat. Cover and let the mussels steam for a few minutes, just until they have all opened, then quickly scoop them out of the pot and set aside to cool. Drain the mussel liquor left at the bottom of the pot into a cup or small bowl, leaving any sediment behind.

Prepare the topping by mixing the breadcrumbs with the chopped garlic and parsley, then a good drizzle of olive oil, just enough so that the breadcrumbs take on a sandy consistency. Moisten with the mussel liquor until it takes becomes crumbly, but not so much that it forms a solid 'dough'. If you've gone too far, you can always add more crumbs. If for some reason you don't have enough mussel liquor, just add water. Season to taste.

When the mussels are cool enough to handle, remove the half of the shell that is not connected to the meat. Cover each mussels with a spoonful of the topping, pressing down just enough for the breadcrumbs to adhere, but not enough for form a solid mass. Arrange the mussels, in a single layer meat side up, in individual gratin dishes (or one large one). Drizzle them with some additional olive oil.

Pass the mussels under a broiler until they are nice and golden brown on top. Serve immediately.