Fry the crushed onion in heated butter until it takes on a light tan and then add the chopped tomatoes, crushed garlic and parsley. Allow the sauce to stew for 15-20 minutes.
Put the well-washed mussels in a large pot with a little oil and the crushed garlic to cook until they open slightly.
Top them up with the wine, some spices to taste and cook until the mussels open completely. Take them out of the shells and add them, along with the wine sauce, to the tomato one. If necessary, add more salt and pepper. After 5 minutes, remove the mussels with tomato sauce from the heat.