How to Make It
Make hazelnut herb butter: Pulse nuts in a food processor until very finely chopped, then pour into a bowl. Whirl shallots and herbs until finely chopped. Add salt, pepper, lemon juice and zest, and butter; whirl to mix. Scrape into a bowl and stir in nuts.
Pour beer into a wide 6-qt. pot and add mussels. Bring to a boil over high heat, covered; then reduce heat to medium and simmer just until mussels open, 1 to 2 minutes, stirring halfway through. Uncover and remove pot from heat. Line a large rimmed baking sheet with slightly crumpled foil.
When mussels are cool enough to handle, discard any that haven't opened. For opened mussels, discard one side of each shell and loosen mussels using a butter knife or an oyster knife.
Set mussels on foil, pushing them into foil a little to keep them from tipping (it's okay if they're crowded). Top each with a small dollop of hazelnut herb butter.
Preheat oven to 450°. Sprinkle each mussel with about 1/2 tsp. bread crumbs, plus salt and pepper. Bake mussels until bubbling, crisp, and lightly browned, 5 to 10 minutes. Meanwhile, line a serving platter with whole hazelnuts or rock salt.
Using tongs, arrange mussels on platter and serve.
*Toast hazelnuts in a 375° oven until the skins split and crisp up and the nut is golden brown beneath, about 10 minutes. Let cool slightly, then rub the nuts together in a kitchen towel; most of the skins will fall off (a bit of skin is fine). When scrubbing mussels, yank off any threadlike tufts protruding from the shell. To make bread crumbs, trim crusts from slices of sourdough, then whirl the bread in a food processor into fine crumbs.
Make ahead: Hazelnut butter, up to 3 days, chilled, and up to 3 months frozen. Mussels, through step 4, up to 1 day.