In a large pot (or the kitchen sink) soak the mussels 10 minutes in enough lightly salted cold water to cover.
In a separate large pot mix the onion, tomatoes, garlic, beer, wine and peppercorns. Place the mussels in the pot and bring to a boil. Cook 10 minutes, reduce heat to low and continue cooking 5 minutes, until mussels open. Discard unopened mussels.