Remove the beards from the mussels, rinse them well, and transfer them to a large saucepan or stockpot. Add the wine, half of the diced onions and the parsley, cover, and bring to a boil over medium-high heat. Cook the mussels just until they open wide, shaking the pan regularly so they cook evenly. When the mussels are cooked, remove them from the heat. Transfer the mussels to a large bowl. Strain and reserve the cooking liquid - there should be about 2 cups (500 ml). If there is not quite 2 cups, simply add water to make up the difference. When the mussels are cool enough to handle, remove them from the shell. Reserve the meats and discard the shells.