Heat a Dutch oven or other large pot over medium heat; add the oil. Cook the onions and garlic until translucent and tender, about 5 minutes. Stir in the wine, scraping up any browned bits from the bottom of the pot; add the mussels. Steam the mussels, covered, over high heat, about 2 minutes. Stir in the butter and sprinkle with salt. Cover and cook until the mussels open, about 2 minutes. Sprinkle with the parsley.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.