1 cup fresh white breadcrumbs (from about 3 [1-inch-thick] slices of bread)
1 tablespoon finely chopped fresh Italian parsley leaves
1 medium garlic clove
finely chopped
Kosher salt
Freshly ground black pepper
Instructions
Heat the oven to broil and arrange a rack in the top position.
Place the mussels and water in a large straight-sided frying pan over low heat and cover with a tightfitting lid.
Cook, shaking the pan occasionally, until the mussels open, about 15 minutes.
Remove from the heat, uncover, and set aside to cool slightly, about 5 minutes.
Pull apart the mussel shells and discard the meatless side of each, along with any beards (the beard is the thread that runs along the side of the shell; remove it by pulling it toward the hinge of the shell).
With your fingers or a teaspoon, carefully detach the meat from each shell, place it back in the shell, and place the mussels on a rimmed baking sheet; set aside.
Rinse out and dry the frying pan.
Place it over medium-low heat, add the butter, and heat until melted.
Add the breadcrumbs, parsley, and garlic, season with salt and pepper, and stir to combine.
Remove from the heat.
Using a spoon, evenly divide the breadcrumb mixture over the mussels.
Broil until golden brown and crunchy, about 1 to 2 minutes.