First, place the mussels in a large bowl with cold water and cover. Then let them soak for about 20 minutes to remove any dirt or sand. While you’re doing that, heat 3 tablespoons of olive oil in a pot over medium-low heat. Add 5 cloves of garlic, and saute for one minute, but do not let the garlic get brown. Next, add the 6 chopped roma tomatoes (and a little onion if desired) and cook it until it’s all almost tender. Then pour in two cups of white wine, and stir in some parsley and butter (optional). Bring the mixture to a boil, and allow to boil until the liquid has reduced by half, typically about 15 minutes.
Now you can add the mussels to the pot, cover and allow to cook on low until the shells are opened, about 10 minutes. Transfer the mussels and sauce to a large serving bowl, discarding any unopened shells.