Place mussels in a large bowl with cold water to cover. Let them soak for about 20 minutes to remove any dirt or sand.
Heat olive oil in a large saucepan over medium-low heat. Add garlic, and fry for 1 minute, but do not brown. Add the chopped spring onion and tomatoes. Cook until almost tender. Pour in the white wine, and stir in the butter and parsley. Bring to a boil, and allow to boil until the liquid has reduced by half, about 15 minutes. Season with salt and pepper to taste.
Add the mussels to the pot, cover and allow to cook until the shells are opened, about 8 to 10 minutes. Transfer the mussels and sauce to a large serving bowl, discarding any unopened shells.