Cook shallots in butter with 1/4 tsp salt in a 5- to 6-qt heavy pot over medium heat, stirring occasionally, until softened, 3 to 4 minutes. Add peppercorns and cook, stirring, 2 minutes, then add wine and bring to a simmer. Add mussels and cook over high heat, covered, stirring once, until mussels just open wide, 5 to 7 minutes. Discard any that remain unopened after 7 minutes.