In a large pot (or the kitchen sink basin), 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.
