Serve this dish with toasted bread to soak up the juices. To debeard the mussels, grab the beard with a kitchen towel for a better grip.
Ingredients
1 large shallot, finely chopped
1/4 onion, finely chopped (about 1/3 cup)
1/4 cup dry white wine
3 1/2 tablespoons unsalted butter, cut into 1/2" cubes
1 tablespoon white wine vinegar
4 1/2 pounds mussels, scrubbed, debearded
Freshly ground black pepper
2 tablespoons chopped flat-leaf parsley
Special equipment: Cheesecloth
Directions
Place shallot, onion, wine, butter, and vinegar in a large heavy pot. Add mussels, cover, and bring to a boil. Cook, gently shaking pot once or twice, until mussels open, 3-5 minutes (discard any mussels that don't open).
Set a strainer lined with a double layer of cheesecloth over a bowl; set aside. Using a slotted spoon, transfer mussels to a large bowl. Pour mixture in pot through prepared strainer. Season broth with pepper and pour over mussels. Sprinkle with parsley.